In a situation that someone wants to date you but you don't have mutual feelings toward them. I would let the guy down in a way that wouldn't hurt his feelings. For example: I like you a lot but I'm just not ready for a relationship. Also, what's the best way in which you would like to be let down/rejected?
I tell guys I don't want a relationship, but it never works. There's always all this "why" crap and all this trying to talk me into it. And nowadays guys will ditch the pretense if you say you don't want a relationship and start asking about just having casual sex, which is really what they want 9 times out of 10 anyways. So now I have to come up with something that will get rid of them on the spot without getting cursed out because I don't want to date them or have sex with them.
I mainly have the above problem at work when I know I will see the guy again. Otherwise, I just give them a wrong number or give the right number and never answer or respond. But I keep a biotch face and don't act that friendly, and it keeps guys at bay for the most part when I'm not interested. Don't ever say "I like you a lot," either--it gives false hope for the future.
I don't want to be rejected--that's why I never ask anyone out. If I ask someone out, it has always been a friend whom I could totally tell liked me.