Alien
Use hit and run tactics with pounce
Aliens have devastating close range combat abilities -- their claws, tail, and jaw attacks -- but absolutely no medium- or long-range engagement abilities whatsoever. If you're an Alien, you have to close quickly, do your damage, and then get out (in case you your victim is still alive or his friend are nearby). This where the pounce comes in -- the pounce is a massive, fast jump. The Alien must take breaks between pounces, so pounces need to be timed carefully in order to prevent the Alien from being stranded under sustained fire. When a pounce actually lands you on top of an enemy, it does devastating damage, usually completely obliterating the corpse -- the drawback here, however, is that it leaves you no corpse to mutilate for health. Pounce in, rip your enemy to shreds, and then pounce out.
Circle strafe
The Alien claw and tail attacks are extremely effective, but only at close range -- but at close range, the human and Predator weapons are even more deadly. The key to Alien survival in a melee is the circle strafe, which involves sidestepping around one's victim, but moving forward at the same time so that one moves in a tight circle with the target at the center. Due to the Alien's greater speed, a good circle strafe with claws or tail blazing can make it very difficult for a human or Predator to return fire, since a rapidly moving Alien at extremely close range is a hard target to keep a bead on. When in close, circle strafe around the enemy, claws and tail blazing. Your victim can only survive a few seconds under such tained attack.
Stun your victim and then circle around to rear
A charged tail attack will stun a human victim (for a Predator, a pounce attack is required) for a few seconds; during this time, their vision is distorted and their sensitivity to movement is considerably reduced. The best way to take advantage of this is to stun your prey with a charged tail attack, circle strafe around to the rear, and then claw attack (or jaw attack) until you take the prey down. Circling around to the rear is the best way to ensure a clean kill; due to the reduced sensitivity, it is much harder for your victim to track you as you circle strafe around. Stun them, circle around to their rear, and have at them.
Think in three dimensions
Adult Aliens can wallwalk -- don't forget this. This is an ability that none of the other species have, and which even intermediate players frequently forget. Crawling up from the bottom of scaffolds or dropping down out of vents behind an enemy not only gives you an easy kill and a corpse to chew on, but also has the added benefit of scaring the out of the other players. You're an Alien -- act like one.
Two words: Be sneaky
An Alien's greatest assets are its intrinsic abilities. One of these abilities, which is rarely mentioned during gameplay, is the mythos from the movies that the games are based on. Aliens are scary, and an Alien player can take advantage of that. There's nothing more effective at keeping your enemies -- particularly humans -- on their toes than giving them a good scare. Take advantage of wallwalking to get into areas that your enemies won't expect; sneak up on them from above or below when they are unsuspecting. In lifecycle games, there's nothing more demoralizing than a well-timed facehug. Use the Alien's innate abilities to keep your enemies off guard.
Attack opponents simultaneously from opposite sides
Their short-range engagements mean that Aliens need to get in quickly, do their damage, and escape to cover. This is made much more easily when Aliens engage in coordinated attacks. Humans and Predators have very powerful weapons that can make short work of any Alien that's directly in their sustained line of fire, so it's best to attack from multiple angles (including above and below!) simultaneously. If one distracts enemy fire, another can get in behind the victim for an easy kill. Coordinated attacks with decoys and hunters will make short work of the enemy.
Move in batches or with adults (lifecycle)
In lifecycle games, facehuggers are the key to the enterprise; all the Aliens start as one, and they must facehug victims in order to graduate to adulthood. Power in numbers is always a factor in gameplay; a lone facehugger is easy for a Marine to dispatch, but a batch of several, or a facehugger and a few adults is far more difficult. Even with a large grouping of alert humans, a swarm of facehuggers is bound to land a few facehugs. Swarming in groups of facehuggers and adults to draw fire is most effective.
Strike while your victim is otherwise occupied (lifecycle)
Facehuggers are small and can easily slip into and under a skirmish without being obviously noticed, particularly by humans who lack an explicit vision mode for detecting Aliens. The best time for a facehugger to strike is when your victim is occupied, either because he's battling an adult Alien (or even trying to pick off another facehugger) or simply another species. (Marine vs. Corporate skirmishes are often the best, since human weapons are so powerful the attention of both will be fully concentrated on the other.) Wait until your potential victim is caught up in a skirmish with someone else and then move in for the kill.
When waiting to chestburst, plan your escape route (lifecycle)
Chestbursters are the most vulnerable form of the Alien lifecycle -- they are slow, very weak, cannot wallwalk, have almost nonexistent offensive capabilities, and make a distinctive noise when they emerge, alerting humans and Predators to their presence nearby. After you've facehugged a victim and are waiting to chestburst, plan your route of escape -- take a look at your surroundings and decide on the best place to go. Finding ledges or ductwork (the charged jump can get the chestburster a little higher, although it can only jump straight up) are your best bet. When you're waiting to spawn as a chestburster, starting planning your escape to a quiet area where you can safely grow up to be a strong, healthy adult killing machine.
As a chestburster, hide in ammo boxes if no other cover is around (lifecycle)
Chestbursters, when hunkered down, can hide almost entirely within the Marine and Predator ammunition boxes -- chestbursters are at ground level and can walk right through them. While not perfect, it can make you much harder to see, especially when you're on a small level where there is a lot of hectic, close quarters combat. Of course, when you advance to the full adult stage, you will be instantly visible. It's not a perfect camouflage, but a chestburster hiding in an ammo box can last a lot longer.
Mutilate corpses for health after securing an area
Adult Aliens gain health by mutilating corpses with their claws (and a jaw attack on a corpse for drones and runners). Once you've made a push into a new area and taken out the humans and Predator, backtrack through your spoils of war and mutilate the corpses lying around for health. Taking time out to do this while you're still under fire leaves you can easy target, so be sure the area is safe before proceeding. Heal up by mutilating corpses between skirmishes; you'll need it.
Attack Predators when they are hunting another species
The Predator's vision modes allow him to see his prey easily regardless of what else is going on. But these vision modes have a serious disadvantage: each vision mode is good for detecting one species type (human, Alien, and Predator), but is very poor at detecting the others. Take advantage of this failing; when you see a Predator pursuing a human (or even another Predator in free for all), chances are very good that the Predator is in the wrong vision mode to see you. This gives you the best opportunity to take him out. When a Predator is in a skirmish with a human, not only will he be distracted, but chances are he'll barely be able to see you at all.
Human (Marine and Corporate)
Engage enemies at a distance
Humans have the most powerful weapons and equipment of the three species, but as compensation they are physically the weakest. Any sustained attack will bring a human down quickly, so the key is engaging enemies on your own terms -- at a distance, where they (particularly Aliens) cannot deal much damage to do you -- yet. Bring your weapons to bear on targets at medium and long range.
Retreat while firing at charging Aliens to widen gap
The Aliens' weapons are short range -- claws, tails, and jaw. A charging Alien is serious threat -- if it reaches you, it will make short work of you. Learn to backtrack when an Alien is rushing you; this will help lengthen the time it takes to reach you while you can fire full-bore. Pull back from charging Aliens to gain a few extra seconds of sustained fire.
Stick together
There is strength and numbers, and for humans this is no exception. Human weapons are very powerful, but humans themselves are relatively fragile. Further, humans can only detect enemies at range when they are moving -- and only when their infrared vision mode is not enabled. It's vital for humans to move in groups and cover each other, particularly in an environment where Aliens and Predators can come from any direction. Watch each other's backs -- because your enemies certainly won't.
Reload when things are quiet
Make sure you keep your weapons at the ready; there's nothing worse than having to reload at an inopportune time, particularly with enemies as powerful as Predators and as fast as Aliens. When things calm down, reload.
Do quick reverses to get complete motion tracker coverage
The motion tracker is the human's secret weapon, but it only covers the 180 degree forward arc in front of its user. It is vital, then, to make frequent quick reverses (and hold them for a moment so that your motion tracker can get a full sweep) to "clear your baffles." Especially with sneaky Aliens and cloaked Predators, you need to constantly watch your six. Reverse, wait a tick for a motion tracker sweep, and then reverse again.
Predator
Cycle through your vision modes frequently
Perhaps the Predator's greatest weakness is the polarization of the vision modes. The thermal vision mode is excellent and spotting humans, but almost useless at detecting Aliens. Conversely, the electrical vision mode highlights Aliens with ease, but makes it very difficult to discern humans. The key to playing on a server where there are both humans and Aliens is to frequently cycle through vision modes. Keep in mind that while you can see one species, you cannot see the other.
Retreat to use the energy sift and medicomp in protected areas
Playing as the Predator means managing your energy reserves, and the energy sift gives you energy for free. The only problem is that it makes a noise and light show that alerts nearby enemies to your present. When things quiet down, retreat into a secluded area and use the energy sift; you are injured and have time, then use the medicomp (which will significantly drain your energy) and use the energy sift again. The Predator is a hunter, so it's important that you be able to get quiet times to heal and recharge. Find time in your hunting to take time out to heal and recharge.
One word: Hunt
Predators are extremely powerful, but in close quarters against an Alien or at medium range between an alerted human, it doesn't take long for a Predator to fall. The Predator's assets are best tuned toward hunting, not getting involved with free for all skirmishes. Predators should predate; hunt your victims. Take out your enemies from a distance when they barely know you're after them. Predators are hunters. So hunt.
Self-destruct when things are hopeless (Primal Hunt)
The Predator's self-destruct ability in the mission pack is only helpful when his health is below 10%, so it's a weapon of last resort. The self-destruct timer only lasts a few seconds, however, so it's usually good for taking out a few enemies when surrounded.