From my own experience, I would feel some crushes that would come back, and I'd get preoccupied with that person. Either the feeling would pass, or if not, I'd try to find out what that person was up to (nothing stalkerish, just ask a friend, or check facebook), and that usually helped kill the crush.
For example, I had a big crush on a coworker years back that was unrequited. Off and on it would come back and bug me, until I found out she was working at home depot, and was looking a wee bit chunkier.
I quickly realized I hadn't missed an opportunity so much as dodged a bullet, because I have no time to waste on lazy, unambitious people.