The Ironic Catholic has produced a handy hints guide on how to behave at Mass, and avoid those embarassing faux pas. I think one of my favourites was:
8. Yes, the Holy Eucharist is a participation in the Heavenly liturgy, as we transcend time and space, singing with angels and saints. Ergo, it is politic not to look bored. However, if you are too enthusiastic, remember you will be accused of Pentecostalism. You are hereby challenged.
And the ever-helpful Curt Jester has offered a few more guidelines, including:
Asking the person who just came out of a confessional both after a long time in confession "What the heck did you do?" is not considered appropriate.
Continuously pointing at your watch for the priest to see during the homily is considered rude.
Read and learn!