- Read the FAQ for the fandom/community, if any. Regularly.
- If you only have "SQUEEEE!" to contribute to the discussion, don't. [Edit: If you're going to go "SQUEEEEEEEEE!" fine. Please also contribute coherently once you're capable of it. ]
- Namedroppers shouldn't.
- Spelling, grammar, punctuation, and coherent train of thought count.
- There is no harm in lurking for a bit to sound out a community and get a feel for the usual flow of things before leaping right in.
- It's not necessary to contribute to every single thread of discussion, especially if you're not really sure what's going on, but need to make your presence known anyway.
- Knowing every possible little bit of trivia related to the object of your fangirl/fanboyishness looks more like creepy stalker type than pleasingly devoted fandom -- unless the object of your obsessive fannishness likes stalkers.
- Same with the thing where you're not able to talk about anything but your adoration of $FAN_TARGET.
- Oh, and that thing where you think that everything that $FAN_TARGET has done is wonderful, even if you'd think it would be crap if it were done by anyone else? Also not attractive.
- Did I mention how obnoxious it is when you let $FAN_TARGET's taste in something (music, media, friends, fashion) dictate yours? Especially where you'd hate it if it were endorsed by someone who $FAN_TARGET is having a running feud with, but love it if $FAN_TARGET said that they liked it, the very next day?
If you'll follow a few of these rules, and, you know, think for yourself and become an interesting person outside of your screaming fandom, maybe people would actually think you had a life.