Come prepared to capture your best performance. That means rehearse, rehearse, rehearse. The studio is not a place to discover what you want to record. It’s simply too expensive. You can do that by exploring your material fully on your own, making decisions about the structure of your music well in advance of your recording date.
A special note: try to rehearse your material at half, or better yet, quarter volume. You may discover that some things have been hiding underneath the cloak of all that volume. The very best music derives its integrity from precision. Many times volume simply hides a whole lot of imperfections.
Make sure your equipment is in great shape. New strings, fresh cables, effects units with no buzzes or grounding problems. You don’t want to spoil a great track with unwanted sounds.
You may want to bring snacks or drinks for breaks. You want to keep fresh and your energy up. Tanglewood has a great break room just outside the studio for catching a quick pick-me-upper. The refrigerator and soda machine are always available to you, as are the number of nearby take out and delivery joints.