4. If you come to Mass tired, don't slump disrespectfully. Depend on the skills that got you through high school. At least hold your head in your hands in such a way that you look reverential.Tom's favorite:
10. When all else fails, remember the secret Catholic motto: "I can sing quieter than you can sing."My contribution would be: If you are kneeling in prayer avoid breathing down the neck of the person in the pew in front of you (discreet throat clearing is allowed so the person knows you're there and might lean forward slightly.)
This has spread like wildfire so be sure to check IC's comments boxes for other good additions, as well as her links at the end of the post to other bloggers who have taken up the list.