Everybody in the Studio is on Your Side
From the producer to engineer to the schoolkid on work experience, everyone wants to have a good session. They are not there to judge nor criticise, they are there to help. Keep the session friendly and positive. Of course, there are arrogant producers, grumpy engineers and incompetent assistants out there but don't let yourself be the one who makes your session an uncomfortable experience. Be open to ideas that might makes your material better. It's possible these guys know what they are talking about.
Preparation, Preparation, Preparation...
It should go without saying but you would be shocked by how many people turn up at sessions with half-written songs and equipment that doesn't work. Rehearse your material and make sure the parts fit. It will save everyone's time and your money. Set up your instruments beforehand. Bring tuners, straps, leads and batteries. Don't assume that the studio will have one.
Everybody Needs to Eat (and Drink)
Allow for opportunities for everyone to take on food and water, especially during long sessions. You may not notice that the engineer has been glued to his chair for the last five hours. When he suggests taking five minutes, embrace it. A break will improve everyone's attention and enthusiasm... and the best thing for tired ears and frayed nerves is the sound of a boiling kettle.
Mixing is not Free...
Neither is preparing stems or doing files transfers. If it takes time to do then that time will cost you. Your friendly engineer will likely be giving you free time for things you don't see already. Yeah... a living wage for engineers!
Blow Your Own (and everyone else's) Trumpet
When you drop your new product, advertise the news! Put up some pics and video of the process... tag the studio, producer or engineer who made it possible. It will give everyone involved a warm fuzzy feeling.