Alien Tactics

Spread out across the map
Humans are almost always limited to a single base location. Aliens are not. Eggs can be located all over the map (preferably in dark ceiling corners). This gives aliens the ability to surprise humans by attacking from multiple locations. In many games, if the humans do manage to destroy the Overmind, a new Overmind can quickly be built elsewhere (as long as there are surviving eggs). But don't egg spam – on a 100 Build Point server more than four eggs leaves too little for defense.

Using the booster
Proper use of a Booster gives the aliens a huge advantage. Boosted aliens heal faster, and its poison causes humans additional damage. A Booster should be somewhat forward, along the main attack vector toward the human base. Ideal Booster placement is directly below a ceiling egg, so dretches will drop onto it and become poisonous right away.

Distracting Idiot Humans
Two of the alien classes (Dretch and Marauder) are extremely mobile. They can play an important role in drawing humans' focus away from more important objectives (pressing the attack or defending against more powerful aliens).

As a Dretch, it's pretty fun to run zigzag all over the walls and ceiling while humans waste their ammo. You'll be amazed how crappy Human marksmanship is.

A single Marauder can keep several humans busy for a long time (and maybe kill one or two of them). This really helps keep the pressure off aliens, giving your teammates time to build, respawn, and position themselves for attack. An Advanced Marauder on a hopping spree with its lightning attack can be more effective than a Dragoon.

Dretching
Good dretching is arguably all that is necessary to be an extremely effective Alien player - if they can score headshots, dretches are extremely powerful against stage 1 and 2 humans and can allow aliens to reach Stage 3 quickly with a healthy stockpile of Evolution Points to become powerful Tyrants.

Basically, attacking humans with a Dretch has two important components: closing (maneuvering and evading until within striking range of the Human) and scoring headshots (maximizing damage once in striking range). Here we will address each component in turn.

The most obvious advantage humans have over aliens is their ranged attacks, but a good Dretch can nullify this advantage by closing fast on Human players. In close quarters, the seemingly weak Dretch has an advantage over most better-equipped humans - so half of being deadly with a Dretch is merely putting yourself in this advantageous position. Here are some tips:
 * The most important use of your wallwalk ability is to evade Human fire. Walk on walls and ceilings to confuse and disorient humans until you are close enough to damage them.
 * Rather than going to the humans, you can wait for them to come to you. Wallwalk also comes in handy in this situation - hide on a wall or ceiling near a corner or door, wait for a Human to walk past you, and then drop down to surprise him from behind or simply attack him right as he comes into view. Note that this will not work as well against helmeted Stage 2 humans, who can detect your presence with their Radar. Smart humans will also check their backs after passing suspect doorways, so strike fast.
 * In nearly all circumstances, avoid charging forward on the ground against a backpedaling Human. No matter his health level, weapon, or Armour, he will have an easy time killing you before you can reach him. Dretches are fast and maneuverable, but this tactic will nullify those advantages and leave your weakness, a low total HP, open for exploitation.
 * Dretches get a slight speed boost every time they jump. In a long area (the plant room in Tremor comes to mind) it is possible to build up enough speed that any Human would have a very hard time realizing your presence before he is taking damage.
 * You should generally avoid jumping, however, as it makes an easily recognizable noise that will allow helmetless humans to detect your presence and either steer clear or improve their initial aim.
 * Avoid attacking grouped humans. Even the best Dretch will probably not survive against two unarmored, Rifle-wielding humans (although he may kill one), and it is near impossible to inflict any damage against a group of three or more humans. Rather, you are better off identifying isolated humans or using extremely cautious hit-and-run tactics to pick them off one by one.
 * Be especially careful around doors and choke points. Humans like to camp these to rack up kills, and they will succeed if you try to exit on the floor, right into their line of fire. Instead, try to find an alternate route to their position and flank them, surprising them from behind as they wait for you to come from the standard direction. If you absolutely must exit a door that is being camped, trigger the door first and then, once it is open, skitter out along the ceiling or wall.

Once in range, your Dretch can really show off its natural talents. It is absolutely essential that you aim your attacks at the Human head, which is twice as vulnerable to damage as his body. A Dretch headbite on a helmetless human does 96 HP of damage, which is quite devastating to say the least. Below are some important elements to scoring headshots. Against helmeted humans, your headbite is significantly less effective but can still be devastating. Because they will take more hits to kill, it is important that when you are in close proximity to a helmeted Human you can evade his fire long enough to get plenty of headbites. It is helpful to try to get behind a helmeted Human at some point during your attack.
 * Although dretches damage humans simply by “touching” them, your attack is still directional - it will inflict damage in the direction that you are looking. Keep this in mind especially when you are attacking humans from above, as it is possible to sit on top of a human's head without injuring him, if you are not looking downwards.
 * Considering the Dretch attack is directional, the most effective method of getting headbites is simply to get near a human's feet and then look up - way up. Imagine that your Dretch is actually holding a massive stick, and you mean to put a human's eye out. If you aim your stick right up at Human head level, you will do some serious damage (provided you are close enough to his feet).
 * The best way to get used to headbiting in this manner is with friends on a private server. Then you can move on to playing against noobs until you really have the hang of it and can give it a go in serious play.
 * While in close proximity to a Human, it is much easier to maintain your bearings if you avoid jumping and instead just move around on the floor. As long as you do not move in a straight line, the human will have a hard time aiming down at you while backpedaling. You will still be able to inflict a lot of damage from the ground if you master your upward aim.
 * Listen closely to the sounds that your victims make - you can tell if you got a headbite by the level of pain in a human's groan. Keep in mind that, if you detect a headbite, your victim is down to 4 HP and may be worth a suicidal charge to finish off.
 * Once you reach Stage 2, hassle your builder to put up a Booster (or do it yourself) and then use it regularly. Poison forces humans to use their medkits, which makes hit-and-run style attacks more effective at stalling large Human rushes. More importantly, though, the Poison will do the remaining 4 HP of damage to a helmetless, headbitten Human before he can even react - meaning that dretches can effectively one-hit kill a Human at Stage 2. Yes, in terms of offensive power against helmetless humans, this makes them as mighty as dragoons and tyrants.

Alien Building
Aliens may spawn as a Granger (available at Stage 1) or Advanced Granger (available at Stage 2) in order to build a base.

The alien building ability is rather limited at Stage 1, and expands during Stage 2, where it becomes possible to build items such as a Booster and build on walls and ceilings.

Acid Tube 101
It's very important to realize that Acid Tube damage doesn't come from the acid spray - damage is calculated by a human's proximity to the tube itself. Sticking tubes way up high on a ceiling to rain acid on those below looks great, but does little or no actual damage.

The best spot for acid tubes is on the walls and ceiling around a door (roughly at Human head height). With a Trapper on the ceiling above the door.

Killing Human Structures
Humans depend on their bases. From a new player's perspective, it appears that dragoons and tyrants are the only serious threat to bases. This is far from the case.
 * Dretches can kill turrets by circling. Dretches move faster than turrets can track. By running in a circle around a turret, a Dretch can destroy it. This requires the turret to be in a place by itself, with enough space around it to run. In some rare situations, it is allegedly possible to destroy two turrets placed very close to one another, as they will block each other's line of fire.
 * Turrets also have a limit to how much they can “depress” or aim downward. If a turret is at the edge of a raised surface, a Dretch can crawl up the side and damage the turret from below without being shot.
 * It's a rare opportunity, but the player who has killed a turret with a dretch can credibly claim to be an expert.
 * Basilisks can kill turrets by circling. A basilisk's “grab” attack slows down a turret's tracking rate to something like molasses. A few swipes while circling, and that turret's done for.
 * Advanced marauders' lightning attack can seriously damage structures during a hopping spree.
 * Advanced Grangers can shoot spitwads of creep that minimally damage structures, but can immobilise turrets. This tactic is almost never employed by experienced players, and most teammates will resent the presence of a Granger in any sort of offensive position. Partly because the damage they do is insignificant and contributes little to an assault, but mainly because grangers can actually have a negative impact on the team. They are less mobile than any of the attacking classes, and tend to block other aliens that are trying to retreat or maneuver. No one wants to lose Evolution Points because a Granger could not get out of their way fast enough in a cramped area.
 * The one potential use for a Granger in combat is providing bait for an ambush, another use of the Granger is to stop chasers. this is done bye shooting the chasing human with spit and slowing him down greatly. Although this probally means that the Granger is done for, this tactic can save Tyrants and larger aliens from death
 * Dragoons (and Advanced Dragoons) can use their jump to get over lines of turrets to do damage to the vulnerable human Armoury, Spawns, and especially Reactor.
 * Reactor jumping is a very common strategy for expert players when there are only a few Human players left guarding their base. If the turrets are far enough away from the rest of the Human base, it is best to take out the spawns, then Armoury first, since there is no “Base is under attack!” warning for these. Otherwise, it is best to jump straight to the top of the reactor and take it out: in almost every case, the turret fire will NOT REACH THE TOP OF THE REACTOR. Once the Reactor is down, make sure to let your teammates know!
 * If you can get on the top of the humans' Reactor - do so! You can attack the reactor from above (look down and chomp/swipe/claw). You'll be harder to hit, you'll take less damage from the Reactor itself, and on some servers where Friendly Fire on structures is on, humans may destroy the target themselves.

The "block & wait" strategy
The aliens' most secure strategy is to contain the humans' assault until Stage 3, when tyrants and advanced dragoons show up. At this point, the momentum shifts to the aliens, who can now present serious threats to the Human base. Only good defense and rapid rebuilding of structures can save the humans. Once Sudden Death occurs, the aliens have even greater advantage.

The main tactic for implementing this strategy is a single Granger who stays in the base and works on beefing it up. All the other aliens focus on owning the halls and keeping the humans bottled up.

This strategy is most effective on maps that favor humans (ATCS, Tremor).

The "draw-em-out" strategy
This requires a map that favors aliens (Arachnid2, Nexus6, Niveus, Karith). This strategy is more complex, and also more fun.

This requires multiple advanced grangers (2 or 3), who roam the map placing eggs in hard-to-reach spots (on the ceiling, usually).

The idea is to be able to attack scouting humans from behind, and to easily relocate the Overmind if it's destroyed. Most human strategies assume a path to the default Overmind location, and many humans aren't flexible enough to accommodate a change in destination. The weakened human defenses are easy for dragoons, marauders, etc. to beat down.

Go up!
You'll probably see this repeated in other places, but it's worth mentioning here. Humans have a real problem moving vertically (they can't walk on walls). And human grenades are hard to toss upwards. Aliens should exploit this weakness by placing eggs, acid tubes, trappers, pretty much everything on the ceiling and walls.

Beware
Tesla Generator

Marauders
Marauders and Advanced Marauders should be used instead of goons, in my opinion. This is because:
 * a) They are harder to hit.
 * b) They can kill multiple humans where dragoons can only hit/kill 1 or 2.
 * c) They can easily get over turrets (though not quite as easily as goons), but are harder to hit when on top of the Reactor. See Reactor Jumping.
 * d) Like dragoons they are weak against tesla generators, but marauders are better against turrets as they can circle them.

Note to Nubs!
The Trapper is useless on the floor. Don't be a nub; put them on the walls/roof instead!