Let me say up front that this information is for beginning voice over talents as more experienced ones will already know this. There is a thing called "studio etiquette." It's sort of the "code of conduct" when you go outside your home to record in a studio with a client or customer.
When in the recording booth, never EVER touch or move, in any way, the microphone. That may get you a major look of disapproval (or scolding) from the recording engineer who is there to set up the session. It's his job to reposition the microphone, if necessary. Microphones can be very sensitive and expensive. So remember, while out in a studio, "Hands off!"
While recording the script and being directed, should you flub a word, don't whine on and on and beat yourself up verbally. That makes you look like a beginner. And you won't win any points with the director(s). Simply listen to the direction as to where they want you to do the retake. This is called a "pickup." Typically, you'll go back to the beginning of the sentence where you made the flub and read the line again. By the way, it's no big deal. Even the best of the best flub a word or two. Nothing to be embarrassed about.
As noted in another post here, arriving late for a session is a HUGE no-no. (No explanation needed for this one).
NEVER make fun of or mock the copy you're recording. The person(s) who wrote it may well be at the session.
After the recording session, come out of the booth, sign any necessary paper work, thank them... and LEAVE. Don't linger with endless chatter. These folks are busy and that's bound to be very annoying to them. It's all about carrying yourself like a pro. It'll go a long way in your voice over career. The business is tough enough as it is. Don't shoot yourself in the foot and look like an amateur. Be grateful for the work and move on.