If you've managed to find yourself in a situation where some male has introduced that inevitably-awkward admission of unrequited love for you, please take heed of the following advice:
If you value the man as a friend, do not, under any circumstances, attempt to save his feelings by telling him "I love you as a friend" or something to similar effect. This will in no way deter his feelings for you--it will merely serve as a signal to him that now is perhaps not the best time to pursue you.
You see, whenever a woman expresses a lack of interest in a relationship, her intended meaning (with few exceptions) is, "At no point in the future will I ever consider you someone I would like to date." The man's actual implication translates to "Sorry, I might want to sleep with you later, but getting into a relationship with you right now is simply not the most convenient time for me."
So when women deny us and attempt to soften the blow with words to stroke our egos, we believe their implication is the same as ours--"I reserve the right to change my mind and have sex with you."
We are, as prevalent pop culture is so fond of indicating to all of you, selfish, infantile pigs.
And your intention not to hurt us will keep us hanging onto your skirts like petulant children.
So what do you say to someone who's just expressed their undying devotion to you? From personal experience, "I'm sorry," works pretty well. "I don't find you at all attractive," would probably do, perhaps with the additional clarification, "What I mean by that is that I will never, ever, under any circumstances, have sex with you." While such statements might be overly cruel, they're far more direct and without pretense than "You're attractive, but I just don't think of you in that way."
Here endeth the lesson.
ETA: I've had enough people question me on why I wrote this--I have a close friend who responded to an admission of undying affection inappropriately. I thought I'd pass on the advice that I gave to her. Everything is going swimmingly with Pinky and me.