How can I overcome feelings of unearned guilt about refusing other people's requests?

Too often, I feel guilty when I shouldn't -- for example, for rejecting unwanted romantic advances or declining invitations to events with family or coworkers. Even though I know logically that I have the right to pursue my own values rather than satisfy the wishes of others, I feel terrible knowing that my actions will disappoint or upset someone else. Too often I succumb to the guilt: I agree to things I'd rather not because I don't want to let someone else down. What philosophical or psychological strategies might I use for dealing with such unearned guilt?