The earlier "no" is the kinder "no"

Seriously, if you think there's a chance you might not come through at all, just say "No" right up front. For the simple reason that a "no" merely closes off one possibility. But alternative possibilities can--an often do--have expiration dates. The longer you delay turning down whatever is being asked of you, the more likely it is that you're frittering away other opportunities for whoever is asking. You only have the right to close your own door, not others besides.