Sit on your final mix for at least a few days. Make notes of any things you want the mastering engineer to pay attention to. If you made vocal-up/vocal-down mixes, note which ones you prefer, but bring them all to the session. You may prefer a different one after it gets mastered, or you may want to lift a line or two from another mix and drop it into the preferred mix. And don't mix until three in the morning before the session! — Paul Stubblebine
Provide the mastering engineer with a compressed and an uncompressed mix so that they know what you are going for, if you enjoy mixing through compression. And be organized; it saves time and money. — Jeff Lipton
Don't allow the Number 1 offender — bad labeling. For example, the band has eight CDs with different mixes or names, and no one remembers which one they agreed to use. This is especially true when the mixes are on CD-ROM, and the band's reference is an audio CD. “Dude, use mix 4.” Then you look at the CD-ROM and they're all named “vox_up,” “vox_down,” “bass_up,” and so on. That's a true waste of money.