Sounds obvious but if you aren't well-rehearsed and confident in playing your material before you come in, you waste valuable recording time, and your own moolah. The ideal situation is that you practice until you can pretty much 'let go' and just 'do it' when you enter the studio. If you're too self-conscious because you haven't practiced enough and aren't feeling 100% okay with the material, that's when self-doubt starts to creep in and second-guessing occurs - the best performance will come when you know your material, and are keeping an eye on what you're doing, and are neither too tense or too relaxed.
Once you've practiced, let yourself relax. This is important - you don't want to get worn out. Don't relax too much - there's a balance - neither too tight nor too loose. No point falling asleep while recording.
Try not to go partying or drinking the night before. This is more important if you're singing - yelling in a bar or any loud room wears out your vocal cords - lack of sleep affects both your performance and your mood. Oh, and hangovers make you sloppy as hell.
Sounds obvious but isn't - eat before you come in. If you eat too much while you record, your body's trying to digest while under pressure. Not a great mix (for yourself or others...).
Again should be obvious but check all your gear one or two hours before showing up. Even if it was okay last week, something - a failing battery, a broken string due to cold, whatever - might've changed. The worst time to find out is when you're in the zone, ready to record. Even worse if you have to drive out to the warehouse to pick up a spare battery or whatever.
Warming up is essential for long vocal takes. I'm a singer and can teach you some if necessary - but your vocal muscles are just like any other muscles in your body - they work better, last longer, if you give them a warmup first.
Bring a water bottle or whatever. You get dehydrated, your brain doesn't function properly. Btw, soft drinks, not so great - your body uses more water processing them than it gets from them. As well as some other things.
If you're more than half an hour late I may cancel the recording session. If you're regularly late I'll start charging for the time. Just a warning.
Even if you're playing garage speed death crack metal, good attitude = good session. Patience with the recording process is good. If you respect me I'll respect you. Etc.
If you want full involvement from me, say so - if you want very little involvement from me - again - say so! If there's a particular sound you're going for, say so - if you're open to any intepretation of the tracks, say so. Figure out what you want- if you don't know what you want... say so!
If you're using a backing track, you have to obtain rights to use it, unless it's public domain or from a royalty-free library - don't grab something off youtube or whatever and expect me to use it. Ask permission.