Get your instruments serviced prior to coming into the studio.
Unless you are a professional technician with years of experience it’s often hard to recognize and diagnose problems on an instrument brought on through live use, temperature changes, transportation etc until they rear their ugly head at the most inconvenient times. Avoid these problems in the studio by getting your instrument serviced by a professional, the cost of the service will pay for itself in the time saved and a much better recording experience.
Old Strings, bad skins and cracked cymbals are a no go.
Good sound starts with your performance and you need a suitable and well maintained instrument to help translate that performance, even the greatest musicians in the world will not reach their full potential with cracked cymbals, old skins or dead strings. Never underestimate the power of some basic maintenance.
Always bring more than you need.
Even something as small as a bad guitar cable can bring a recording session to a screeching halt for hours, these small dead ends can be massively frustrating, they kill the vibe and cost you time and money. Always bring spare Strings, Skins, Sticks, Picks, Cables, Batteries and anything you might need to help the sessions run smoothly.
There is no such this as over prepared when it comes to your songs.
Unless you are a hippy stoner band that wants to record 4 hour jams (which is cool too), you should know your songs front to back and practice your ass off before coming into the studio. The better you can play your parts and can navigate through the songs, the smoother everything will go. Practice alone, practice as a band, play to a metronome, make some low budget demos, prepare lyrics sheets for your producer and just generally try to enter the studio as prepared as humanly possible.