If you are fired upon, move.
If you see a UAV in a red name about you, move.
If you shot a guy while he was facing you, move.
Also move after shooting several people at the same location, players tend to want to hunt for snipers at that point.

Know where to lay claymores and where not too.
Claymores will explode in the direction you are facing when you place them, they kill teammates on TG. Use them well and you can keep an entrance completely blocked from the enemy.

Know who to target, medics and supports (and ofcourse snipers) should be your main targets, but if another friendly squad with armor is rolling in, then aim for the AT's.

Use cover well, most of the maps have excellent places where you either blend in with your ghillie suit or only the end of your rifle is coming out.

If you hear a single shot richochet near you, an enemy sniper has found you, this means he spotted you to his teammates and that you are lit up like a chrismas tree, move.

Don't stick to the same place all the time unless you have never been fired upon there (still move when an enemy UAV is above you).

With the M24, often the time between the first shot and last shot can mean the difference between getting a kill and getting killed. Use a pistol to get that last shot if you are within range for this.

The M24 thing only works on non-body armor classes. The non-body armor classes: A sniper is obvious with his gillie suit, a medic has shock paddles on his sides, an engineer has a mine on his back and a spec ops has remote detonaters on his sides.
If you see this while getting a hit with the M24, switch to the pistol if you are within range for this.

Target squad leaders and spot them whenever you can if you manage to pick them out. Hints to this are who their medics keep reviving and where they spawn. (if not on a flag, and depending on who is dead and who isn't)

Thats about it for now I think.