I very rarely show anymore. About two months ago, I had a problem with that, because I felt that I needed to show so that people respected my bets, but I realized that in the THF games, players do respect my bets, so there's no real need to show good hands, and in the regular games on PokerStars, no one will ever respect my bets, so there's no need to give extra information to the few that actually pay attention.
I feel that since it is much harder to get reads on players while playing online, why give out any little bit more than necessary? The only real tell online is betting patterns, which will be completely exposed if you begin showing your hands. Still, very occasionally I will show in situations that might benefit myself.
I remember yesterday, I was semi-late in a tournament where you have to steal blinds to survive. I was on the button, sort of short stacked, but not that bad, and had KJo. Everyone folded to me, I raised into the blinds which were both big stacks, and they folded. I showed this hand because I wanted to let them, the possible bullies at the table, know that I'm not stealing their blinds, but raising with decent hands in good position. Now, KJo isn't that great of a hand, but definitely worth raising with in that situation, no matter what the blinds were.
Also, read through this article:
http://www.pokertrails.com/articles/showornot.php
I remember reading it a few months ago, and though I don't quite remember if it was particularly good or not, I think it might help.
Show or no show? 
