I tend to make things up on the fly. Usually involving killing people in the face, and murdering dudes. It's difficult to come up with things when my flatmates refuse to play Halo against me. Although the MW2 games end up fairly balanced (and L4D is always hilarious), and allow for much exchange of banter.
Typically shit lines (from various games) include:
"I lost my claymore... *boom* There it is"
"It's like Rock,Paper,Scissors. Only Tank beats everything!"
"Zombies! So many zombies! And... now they're on fire. Who thought that was a good idea?"
"I'm in ur base murdering ur doodz!"
The whole "murdering dudes" thing stuck with me from the ZP of Painkiller. I use it a lot...