Get plenty of sleep the night before.
Lay off of drugs and alcohol for two or three days leading up to the session.
Practice your parts on your own and have them down cold! You don't want to be the one asking to do 10 retakes on your part.
Clean your bass and change the strings a day or two before the session, and work them out enough to get the tuning stable.
If you don't like the sound of fresh strings, change them a week or two before the session.
Bring some bottled water(some studios do not provide this) and some of your favorite snacks. Bring a couple of things to keep you occupied while you are waiting to play, or waiting on others to listen to their multiple retakes.

A book, a Gameboy, magazines, etc.
Have fun! We are going back into the studio in July, and I always enjoy it, although there usually is some boredom involved.
Bring a boom box and a portable CD player with $10 headphones to listen to your mix. Oftentimes something that sounds awesome on $5000 near field monitors will not sound as good on a car stereo, a boombox, or cheap headphones.