This is how I did it in MW2. Firstly, find a gun in each class (except sniper rifles) that you really like the iron sights for and can use as well as a red dot. This frees up an attachment slot for you. Secondly (and most importantly) DO NOT RUN IN!!! If you know someone is coming round a corner, aim down sights at the corner and wait for them to poke their head out, then shoot them. In other words, if in doubt, let them come to you. You usually have all the time in the world. I know someone will call this camping, but it isn't. If they run round a corner and you're already aiming there, is it camping? No. If you must attack, make sure that you know what position the enemies are in first, or you'll be riddled with bullets the second you advance. A recon plane will do, and is a brilliant killstreak anyway. Also, stun/flash grenades are good if you're attacking, but bear in mind that if you chuck a stun in a doorway and it doesn't get anyone, every gun inside will be pointing at that door, so don't use that door.
Hope that helped
Hope that helped