There's this one rule I set a long time ago and have been telling other people who are struggling in appropriate circumstances:
To control a crush, take a deep breath and make him/her a good friend of yours. A crush is a gut feeling, and it feeds on fantasies. By making its target a good friend of yours, you get to know him/her better and develop a balanced opinion about him/her. And the balanced opinion kicks out and replaces the fantasies that your crush feeds on.
About an year ago, I had to give this advice again -- this time, to myself. Not just because I couldn't forgive myself dreaming of another relationship when I was already in one, but also because I didn't believe in long-distance relationships by the time (I had sworn to myself that the relationship I was in would be the last LDR I would ever have in my life).
And it's finally working -- I could verify it by a couple of measures over the past few weeks. It's nevertheless very sad that the electric-blue fire in my heart had to be forcibly put out like that. Anyhow, it's done -- let's hope it's for the better.