How does one best deal with unrequited love?

I am most interested in this from the perspective of someone who harbors feelings for a friend. In particular, how do you "move on"? When I have been in this situation, I have found it difficult to be interested in others I've dated. Is it necessary to distance oneself from the object of one's affection, even if it means to some extent giving up a life-enhancing friendship? What if one would rather remain single than diminish the friendship? Can that be a rational choice? If so, for how long? Does the answer change if the initial rejection was not unequivocal, but based on some possibly temporary circumstances (like a current relationship)?