1) Keep OOC out of the mystery. Don't use knowledge from outside of the RP about player activity, etc, to try and find out who committed a crime: it's much more fun if your character is the one to investigate.
2) Try to avoid anything overly sexual. This group involves mentions of violence, etc, but if you're getting hot and heavy switch to personal messages, please!
3) Have fun!!! Try not to get too upset if your solution isn't right- the real fun of mystery investigation is doing it together with other players!