DO have fun. Remember — this is a community of like-minded people, so ...
DON'T put someone down because their costume isn't top-of-the-line. Somebody may be able to spend only 20 bucks on a costume, but that could mean as much to them as the guy who spent five grand on a costume.
DO wear whatever you're comfortable in.
DON'T judge anyone based on their body type. A person's shape shouldn't determine their choice of costume.
DO feel free to interact with those who are prepared to engage you in conversation.
DON'T assume that the guy in the Jedi outfit knows every last detail about every Star Wars film. And if he doesn't, don't put him down for it. "For me, it's purely the costuming and escapism side of it. I know the basics, but I'm not a big trivia buff," explains Jeff Angel.
DO be respectful.
DON'T gawk, don't stalk, don't offer inappropriate, unwanted commentary, and for God's sake, keep your hands to yourself. Kimmi Markovi: "I did a costume that was a little bit revealing and people kept touching me. It was SO uncomfortable." Jeff Angel's more blunt: "Don't be a cosplay douchebag."