PRACTICE - I can’t emphasis this one enough. Honestly, I don’t mind if you come in unprepared, but in the interest of saving you money, I recommend you really know your songs
METRONOME - Have your drummer (AT LEAST) practice with a metronome or a click track. This is merely a recommendation, but it makes life so much smoother.
Make sure your gear is in good working order - again, really doesn’t bother me, but to save you a bunch of money...
REST! - this is a double entendre as the group (vocalist especially) needs to be well rested. This doesn’t mean don’t practice, it just means don’t get yourself trashed the night before. In addition, I feel Miles Davis put it best: “Don't play what's there, play what's not there.”
Don’t drink - I have no real problem with alcohol, drinking, or even drinking during/prior to/after the recording. I say this because alcohol (usually adversely) effects your voice and your patience. This one is totally up to you.
Come alone - don’t have a posse or a girlfriend or anything like that. Sitting in on a session when you’re not doing anything and have little attachment to the project doesn’t help. No one needs their own Yoko...