Don't speak loudly and in great detail at the gym about the best blow job you ever gave, particularly when there are elderly ladies on the treadmill next to you is one such rule.
I came across another one last weekend that needs sharing: While at a restaurant, do not EVER speak of anything you have seen on a television segment titled anything remotely similar to, "What Cannot Be Unseen."
This should be common sense, but it wasn't to the group of ladies a few tables down from us at a restaurant. After hearing about one such segment (again, in great detail), I had to put down my food because I had lost my appetite. Now, I don't have a weak stomach, so if even I was grossed out, you can rest assured what you just discussed (LOUDLY AND IN GREAT DETAIL) was completely inappropriate in an establishment where eating takes place (or at least DID before you grossed people out).
So, what other rules have you felt compelled to clarify for the morons?