I'll start off with a few of my favorites.
Jumping can get you out of clots, unless you're on hell on earth difficulty.
Stacking pipebombs on the Patriarch do LESS damage than individual ones.
Fleshpounds are weak to explosives, and Scrakes are not.
Got a lot of support damage from some awesome rounds, but lacking in welding points? Easy fix. If you can get it so that only 1 or 2 specimens remain, go to the gate on west london and weld them away from you. Hold m1, and then hit shift+tab, release shift+tab and you will be welding without needing to hold down m1. This makes the Glados door easier too, if you have a medic or someone tanking a clot.
Don't like the music in KF? You can change that! The audio files are easy to change, all you need to do it get a hold of about 48 songs you'd like to be in the game instead, and then convert them using a free audio converter to .ogg files. After that, just rename your new songs to be the old ones, and there you have it! Your own zed mayhem playlist.
And lastly:
Shoot. Move. Communicate.