I try to force my feelings away. Rejecting them, and trying to push them away when I see the person. Also, I sometimes write down my troubles to get it out. Perhaps write down why I should not be in love with the person - not only because he has a girlfriend.
I feel your pain. You just gotta keep yourself busy, and focus on other guys in your life. Don't use anyone as a rebound, but try not to get so caught up in this other guy who's taken that you miss what's right in front of you. Good luck :)
As the previous answers have mentioned, try to forget him, try new activities and distract yourself. If you end up dreaming of him (and you remember) force yourself to erase that dream. What I have found helpful in the past is to read a book (a good one, with a good story line and all) before going to bed so that you'll end up dreaming of that instead of getting distracted thinking of him.
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