during practice, i listen to the other members, lock in with the bass, and listen to the vox. i think about my parts and try out different stuff to see how they work in a song, so it won't go stale for me or the other members.
on stage, i think about food, getting a drink after the set, girlies in the audience, hoping the house kit doesn't fall apart midsong, as well as listening for variations the other members might've thrown in for some reason or another. i find that concentrating too much on what i'm supposed to play on stage, if the hihat is on time and so on, just messes me up. that's what practice is for...playing a song so many times through and knowing it inside and out, with the same members, that one should be able to "be free" enough to toss in variations, yet still be locked in tight as a group.