I've been having that problem, and the solution seems evident to me: Learn how to lie. My friend is a constant liar and I noticed that almost any favors people ask him to do, he'll make up some lie to get out of it. I think he has a good idea, so long as you can make good lies and don't forget them, you won't have to disappoint people.
And for people who aren't friends, dangerous, or wielding a gun, saying "No" shouldn't be a problem.