1. Pick your spots
2. Don't overdo it
3. Don't ruin the joke
4. You're still responsible
Very interesting tsuwm!
My general rule has been Don't use an emoticon in cyberspace if you wouldn't use it in written (pen,paper,ink) letter. This presupposes that the person doing the writing still uses real mail.
It is a rule I have broken from time to time and I thank you for the reminder - I must pull up my socks.