Keyboards -  Keys are the easiest thing to prepare ahead of time. You should be able to walk into the studio, turn on your keyboard(s) and say "this is my sound". If you have an arsenal of modules, use a mixer so you can feed the engineer a stereo output. This will assure you of having the sound you intended, and relieves the responsibility of communicating your sound verbally to the engineer. This will save a lot of time in the end.

If your band is creating midi tracks, preproduction IS A MUST!  Most of the basic tracks can be created before you get to the studio. No matter what type of sequencer you use during preproduction, you can dump the sequences into the computer the tracking studio uses and separate the sounds using their sound modules. In other words, no matter how primitive (within reason) your home studio may be, much of the pre-production completed at home may be used for the session.

Generally, a well equipped studio will have the capability of importing your sequences, via midi, and separating out each part (bass, keys, kick, snare etc.) using their sound sources.

Several points you should keep in mind. Like the live tracking session, you want the songs to have a feel, or a groove. Some sequencers will quantize every note and take away much of the human feel. Quantizing is basically fixing the timing of rhythms to the nearest note. That note is usually assignable (1/8th note, 1/16th note etc.) I'm not saying the tracks should be loose, they should be tight (rhythmically), but maintain a feel. This means either loosen up on your quantize setting, or try to play without quantize at all. Many sequencers allow you to set a swing or human feel in your quantize setting. This often helps add some life back to your tracks.

The beauty of using a sequencer in the tracking studio, is you can change a sound at any time. If you hate the sound of the snare drum that you've been listening to, you can switch it at virtually any time. If you don't like the sounds in the modules, bring in a sample of something that works. Switching sounds is a great advantage and is usually painless. Another point I would like to stress is the importance of live instruments in a track that has been mostly sequenced. Small details like live cymbal crashes, tom tom fills and even percussion parts can make a world of difference in the overall feel of the song. These parts can take what a listener would brand a computer generated track and make it seem to be a live performance.

Laying down live bass and live guitar tracks will also help immensely. Even if you have to pay a session musician to come in and lay down these parts, its well worth the outcome.