Cover
Make use of cover in DOD it is not merely there to make the map look complete it's there to save your life. Don't fall into the peek-a-boo frame of mind, "I can't see them so they can't see me." The chances are that your legs are sticking out or that they're in a window behind you.
You're best off staying crouched rather than prone as your body is kept compact and won't stick out as much. Only use prone to peak over obstacles or round longer ones that will hide your full length it's also good for sniping from windows etc.
You'll soon learn in maps which obstacles make good cover and which don't by the fact you'll keep getting shot/shooting people in bad cover. Also learn which positions can fire at you whilst behind cover and keep a close eye on them.
German Heavy Infantry hides waiting for passing Allied soldiers
Crouching and sneaking
Crouching eliminates almost all sounds made from walking and also when used climbing ladders dramatically lowers the noise made. Crouching will steady your aim for shooting and reduce recoil. Also your stamina level will shoot up again when you crouch so remember to do it quickly to regain any lost stamina.
If you hear footsteps or other nearby sounds crouch and keep listening. Move forwards slowly whilst crouching (sneaking) and keep your ears open. Sneaking is particularly good for sneaking up behind people, especially MGs and Snipers, and allows you to approach undetected for a knife or spade attack. Crouching is also necessary to pass through small tunnels and many gaps in walls; this can often not be done in the prone position. This is due to a bug in the game.
Team spacing
It's a good idea to stick together but keep some spacing between you and the others around you. A group of huddled soldiers is a juicy grenade or automatic weapon target. In the words of Sgt. Horvath (Saving Private Ryan) "One man is a waste of ammo, five men are a juicy opportunity."
Another reason to keep apart and not to crowd into rooms is that doors are made for one man at a time and if a grenade comes through the window no-one will get out because you'll all get caught up at the door. Also if you're spaced and one member gets shot then you have the chance to work out where the shot came from and not get caught out as well in the same burst.
Jumping
I very rarely if ever just press the space bar. Crouch jumping is the only way to go. This (for those of you who don't already know) is when you crouch then jump. This has to be done quite carefully but with a little practice soon becomes second nature and allows you to jump much further than normal.
Every jump you do drains stamina so bunny hopping is definitely impossible. On the plus side if you crouch jump and hold crouch whilst in the air it not only makes you a smaller target but you'll regain stamina whilst in the air. I generally hold Crouch after landing for a tiny bit too so my stamina bar is all the way up again. The crouch jump is also necessary to pass through smaller gaps like windows and with practice you'll be able to jump through windows without touching them, it's purely a matter of timing.
Crouch jumps are a great way to get around corners a well timed crouch jump with good use of the mouse and strafe keys will send you flying sideways around a corner ready to take on any body waiting for you. Remember that when jumping you can still turn around so by using the mouse and strafe keys you should always land facing the direction you want no matter which way you jumped.
Jumping can also be a good way to avoid enemy fire in a small room or even at times resorting to a jump from a window to avoid being shot or blown up is the best bet. Finally on maps like dod_avalanche jumping from the windows doesn't do much harm but can help you take the flags swiftly or back up a team mate on a flag.
