1. Never attack in a strait line
2. Dont build on a populated game unless you know for a fact you know what you are doing.
3. Realize the each evolution's strengths and weaknesses. Tyrants are huge and powerfull plus all aliens around them get a healing boost, but they cant jump worth crap, cant attack bases that are high, are HUGE targets, and are easy to hit. A dretch has a weak attack against armored opponents plus they have extremely low health. But they are small and hard to hit, they are one of the fastest aleins in the game, they can ambush humans on the walls, and they are litterally one of the best aliens for the job if your against an S1 team.
4. Realize that every time you are killed, the other team gets points for upgrades. Known as "Feeding the other team", it pretty much garuntee's you to lose if you constantly try and attack them and die. Because thats how they get powerfull weapons that kill you even better.
5. Turret kills however, do NOT count twords the human kill count. If you KNOW you are going to die, do so by a turret preferably, unless you are against an S3 human team (because by that time it doesnt matter how many kill tehy get, but it still doesnt help to die).
6. Dretches dont make a sound when they walk. As such, its easy to ambush humans and chase them with them if they dont know you are there. When you jump though, you WILL make a sound.
7. ALWAYS aim for the head. If you a dretch, look up and try and get it so the center of the screen if the human's head. A dretch does 94 damage to a human with no helmit on if it hit the head. Also the only time you should really jump is when you are litterally attacking the human. Such as, jump at them in head level to hit a head shot.
8. Search teh forums! There are plenty of help threads reguarding building as an alien, general tactics, do's and donts, among others.