I really like Azuremen's advice.
It was similar to the one I gave to another member as well.(Check this out)
Couple of my pointers:
If they are shooting you in the face, I think you could improve how quickly & accurately you get your gun pointed at your target. You could try what I use sometimes, and that would be walking around with my weapon up. This is easier to do with the red dot. Also, to improve your aim, try a little exercise before you get into a firefight.
Pick a couple, or several windows, trees, whatever can be used as pretend-targets. Then quickly aim at one of them, then quickly switch to the next, next, back to the first. Might seem silly, but I find it a very nice warm up during team deathmatches.
I hope you try some of our pointers, techniques. Let us know if they work, or don't work for you. And don't quit after just couple of matches. I've noticed on CoD4 that some techniques, guns that I tried for for just a little bit, decided I didn't like it. Later on, I find out that I was totally wrong, and should have stuck with them a bit longer.