Quick-crouching
If you're behind a bunker/obstacle/wall where you are partially covered, then you do what is called:
Quick-crouching: Hold down the crouch button and then release for about 1/3-1/4 of a second before pressing down again. This will enable you to get a shot off on your opponent, but the game will not register you on their screen, making you literally invincible for that fraction of a second.
Obviously you need to have some experience before trying this, but when you are playing against good opponents...you better be taking any advantage you can. And of course, don't try this all the time as you will most likely be flanked from the side and killed because you were focusing on practicing. But DO use this when the time is right, or if you're in a desperate situation (e.g. 6 vs. 1).
I might post some more helpful tips, I feel inspired by this thread.