Shorten your stock if you can and keep your gun pushed well into your shoulder and at the ready at all times. Tuck everything in, elbows should not point outwards keep then pointed down so you are as small a target as possible. At corners never poke your head round, just lean out with your gun and start shooting. Don't get pinned down, just keep moving and shooting otherwise you will be stuck until they flank and shoot you from a different angle. Wear neck, full face, head and ear pro...
Look up cornering techniques on youtube, basics like 'slicing the pie' clearing stairwells and room entry, especially if you are going with a buddy to help you
Carry less gear! You want to be able to move fast and quiet. I found even my small chest rig was overkill for CQB - too many mags and ammo. Belt kit is the way to go. Depending whether it's full or semi auto a few midcaps will be better than rattling high caps! You won't need to carry as much as you're probably closer to the safe zone