Me and my buddy mike when we are playing HQ on COD4, usually take it turns to be the 'Keptin'. This entails one of us putting on a upper crust english accent and saying things like 'follow me lads, over the top for blighty' and trying to knife/pistol kill the entire other team, it usually fails miserably, but does allow the rest of our team to actually catch the HQ, more often then not.
What makes it funnier is that mike is a yank, and usually stoned when we play, and i'm british, usually drunk when playing. When we start rolling on a big win streak it's usually celebrated by singing 'baby got back', which usually gets our whole team laughing and messing about.
the other is when sniping and you get the headshot, to shout 'you dirty *****, you love the money shot!'. always amusing
oh, and whenever you are the last man standing on your team, to start saying 'save me jeebus, save me!'