As with Christmas, if you plan to give a Valentine's Day gift, you need to know whether she has someone in her life who's unaware of how she makes her money. If she does, you need to make sure that the gift is something that she can either explain easily or keep a secret.
Two years ago, I kinda felt like Santa Claus on Valentine's Day, and I had a very memorable one. But having just had to throw away my Christmas gifts, still undelivered because our schedules are incompatible, I think I'll sit this Valentine's Day out.
Oh, and as for what the girls can give the guys: That comes on March 14:
http://www.steakandbjday.com/. (Although the few women outside of this board whom I know to be aware of this day think that it's a joke.)