Often times after a show, many people will wait outside the stage door, often for close to an hour for the cast to come out to sign playbills and other posters or artwork pertaining to the show or their work specifically. There is a good way and a bad way to act the stage door. The good way could mean other people let you closer to the front, and the bad way could mean you're shoved to the back.
1. Don't shove. I know everyone wants to get a star's autograph but they'll come to you. If you hold out your playbill or poster, they'll come to you. Don't get stressed and shove everyone.
2. Don't show up with a stack of 10 playbills. If you're waiting at the stage door with 10 playbills to be signed, the logical guess is that you're going to sell them. Just no. Stars coming out to say hi, chat for a bit, and sign your playbill is a gift and a very gracious thing to do. Don't mess up the experience by handing them a stack to sign and then posting them on ebay when you get home.
3. Be patient. Especially if the show has extravagant costumes, it can take time for the actors to come out of stage door. To pass time you can talk to people around you or wait quietly. Don't complain. Everyone is there for the same reason and you're not the only one who cares what time it is.
4. Compliment the performers. If you think someone did well in the show or if you really loved the show, don't be afraid to say so! The actors are likely to be incredibly kind and gracious. Whenever I've been at the stage door I've only had a good experience.
5. Don't be afraid to ask for pictures! If you really love an actor don't be afraid to speak up and ask them to take a picture with you. Generally they'll be happy to pose for a picture. Just don't take 20 as there are lots of people who are waiting for the same thing.
Keep some of these things in mind, and remember to be polite and gracious. It can be a great part of a theater experience!