It can be pretty daunting for a band with little or no experience to walk into the recording studio for the first time. Being as prepared as possible can make the whole experience much more enjoyable for everyone involved in the recording process. Here's some things to keep in mind...
Bring the best instruments/amps you can beg, borrow or steal. A crappy Strat copy that you bought at the pawn shop for $50 might be ok for your bedroom but if at all possible, borrow your mate's original '62 Fender Stratocaster for the session. I'm exaggerating for effect but you get the idea. We have a few instruments here that you're welcome to use but please bring your own too.
Make sure these instruments are in the best condition they can be. Being able to hear in more detail will exaggerate intonation issues, rattles, etc. Have your guitar set-up professionally and re-strung a few days before the session. Don't change strings on the day if at all possible, unless you want to be tuning every five minutes. Drummers - if you are planning on using your kit, please put new heads on all toms/snares. This will make the difference between an ok drum sound and a great one. Don't arrive with the same heads you've had for the last five years and expect to get the drum sound of the century.
Be well rehearsed. The studio is definitely a place for experimentation but it's not the place to be learning your instrument. If you are planning on recording together as a band, make sure you can play the songs tightly as a band. If you can't play the part you'd like to play - simplify. A simple part, well played will sound better that a more complex part played badly every time.
Think about overdubs. Overdubs are extra layers that are added to the basic tracks to give the recording a fuller sound. In the case of guitars, these might be the same part played with a different guitar/amp combo or they might be completely different parts. This is particularly important for three-piece bands.
Arrive on time to your session, within reason. Everyone gets stuck in traffic sometimes but sleeping in and wasting an hour of your day is not a way to get the session off to a good start.
Relax and have fun. Time is money, but it should also be a fun experience. Otherwise, what are we all doing this for?