Don't be afraid to reject a guy. When someone asks you out they're accepting the risks of rejection and are basically saying they can handle it. So quit feeling sorry for them and going out with them just because you don't want to hurt their feelings. It only makes it worse when you tell the truth since it will only make them feel like they had to be a total loser and make you feel sorry about them to be able to go out with you.
Almost every girl I've talked to or gone out with has done this (Not to me of course.) and to be honest its a bit stupid. Also, any "nice guy" would take the time to chase after you but don't overplay the hard to get attitude since it gets tiresome or just makes the guy think that you just don't care about them.
For the people that don't think they've got the looks and charm you can always keep in mind that sarcasm and a silly attitude gets you a long way.
