This goes without saying, in cover you have less chance of getting shot, and getting shot obviously will kill you. Watch out for grenades though.
2. Learn where else to go if you are flushed out of cover by a grenade.
If there is a particular spot which you really like to go to take cover, DO NOT BE TEMPTED TO USE THAT SPOT FIRST. If you're struggling, then you can move to your preferred spot, but remember, the other team will then associate you with that spot, whilst if you are doing well in the first spot and then move, you can easily keep the pressure on the opposing side.
3. Learn where all the medkits and ammo supplies are.
This is very important in fast-paced games such as Team Fortress 2, as often your main weapon will run out of ammunition very quickly. Knowing where the aids are will ensure your survival for a longer period of time.
Crouching will make you harder to hit when behind cover, will often steady your aim and will lower your walking speed so you can position yourself with both precision and accuracy.
5. Gauge the distance between your current cover and cover up ahead/behind you.
Use single player to work out where you can get to from covering areas you use without running out of stamina, and at what time you run out of stamina and thus become vulnerable when you run further.
6. When sniping, wait for somebody to come out of your target spot first, and aim as close to where their head was as possible.
This should make it so that whenever another player comes out of your area of choice you can get a headshot and kill them instantly. If not, then you should only need minor adjustments.
7. Give 'friendly' advice.
'Ooh, that looks like a very dangerous tree that you just shot'. and so on and so forth. Putting people off subtly can give you the edge. Try to play with their mind a little.
8. Don't hack, exploiting is A-OK though.
Hacking generally means you're a bad player and that you need the game to play itself. Exploiting a bug, however, is clever thinking. If you can shoot someone without it hitting them and just hitting their hitbox, then so be it. If you can cancel animations, then so be it.