Although I'm not sure I myself can always follow this advice, I think the most honest thing to do is just to say something like, "I want to help, but that's more than I can take on right now......" The "demanding" approach by someone is the toughest. That kind of thing can really burn me up.

If you don't mind my asking, do you have someone in particular in mind? I'm thinking, maybe, an elderly parent or a child?