Edit: I also think its a good idea to just let the people in your life know that you care about them in general. It doesn't have to be on Valentine's Day or with flowers. If you're thinking of someone next week, or two months from now, shoot them a PM and let them know. It will mean just as much.
Well this is a downer but there were several times in my life when someone was on my mind and I didn't contact them and then they actually died. This has meant I operate pretty much on guilt and if someone comes to mind I'm very careful about not shoving it aside and I do make contact. As soon as I read Candlelight's post about his dad dying I called my dad.