Be tactful. No need for a failure to describe "why", if only the flaws of man. Lover boy and so will be defeated by your refusal, you don't need to finish his insults.
Speak politely point out that he's a good man, and always meet the right girl. But you can't be, because you don't feel reciprocal feelings. You respect and appreciate him as a friend, but between you there is no "chemistry". Apologize, say you're sorry and apologetic smile.
At least, we can say that you are in love with or Dating another guy. If it's actually wrong, you can ask a friend to play the role of your young person. But lie you risk losing the attention of the guys – the news of your pair will disperse among boys. And non-free she meets the little guys.
Sometimes these methods don't help, and the young man continued to insist and do not accept failure. In this case, we can agree on a date, but it is to show itself not with the best hand. For example, if he likes your cheerful disposition, frown, say something, be dramatic. And after Dating tell me you're not right for each other.
What not to do?
Don't try to invent a false case so as not to offend the guy by the waiver date. It may happen that you will meet on the street, or he otherwise finds out about your lies, then it will be even harder. So just tell me what you don't want to go out because I don't feel any romantic interest in him and don't want to give false hope.
If the guy is offended by the refusal
Absolutely no offense is not enough. To failure on your feelings, even polite, always hurt. But you mustn't blame yourself, because you tried to make the rejection smoother. No need to get over yourself and accept against my will for something not to offend the guy. Then you hurt another person – themselves, which is much more serious.