If you are headed to a producers' session, this means you have already cleared the first hurdle--the pre-read. Since you've already impressed the casting director with your first audition, you just need to do well at the producers' session. At the producers' session, you'll meet the people who can actually give you the job.
Know that you're going to the producers' session because you did well at your first audition. The casting director is sending you to producers because she thinks you are a good fit for the role. Feel good about this and let that confidence enter the room with you.
Do not change your performance from what you did at the pre-read. You did well at the first audition. Don't try to muddy the waters by changing your entire approach to the material. Doing so will also frustrate the casting director.
Expect there to be several people in the room. At pre-read auditions, it is quite often just you and the casting director or perhaps the casting director and her associate or assistant. At the producers' session, you can expect there to be between two and 10 people in the room so there is no finite rule for this. If you are attending a producers' session for a television show, you will meet the director of the episode, the writer of the episode and the producers of the show. If you are attending a callback for a film, the film's director, writer and producer will likely be there.
Wear the same outfit you wore to your pre-read audition. It worked the first time. It'll likely work again.
Incorporate any advice the casting director gave you in your pre-read. Sometimes casting directors will give you redirection in your first audition. Heed their advice as they have special insight into the show and what the producers like to see.
Expect to be taped. Most producers' session feature a camera taping your audition. Ignore the camera completely and perform your scene with the reader. The person operating the camera will make sure they have you covered. If all the necessary people are in the room, the camera is usually just there as a backup. If all the people are not able to attend, then the sometimes use the tape to cast the role.
Relax and have fun. Try not to put too much pressure on yourself. You'll likely attend many producers' sessions throughout your career.
Greet everyone in the room politely when you enter.
Never apologize or make excuses in a producers' session. This only highlights what you think you did poorly.