Our local grocery store has *three* doors, spaced about 20 to 30 yards apart, and it's not unusual to see someone with a bucket at each one, at the same time. Back before my spine grew into place, I'd been known to tell them "I'll get you on the way out" when I'm going in, then exit through another door and say, "Oh, I got y'all on the way in."
I've found my best response to be "Not today," with a smile, and maybe a "good luck." If they persist, I'll walk away, and if they're really pushy, I'll leave and promptly call the store to let them know their charity people are being pushy towards customers. Few persist after the "not today," but I think confidence in your answer means a lot. They can sense if you feel guilty walking by, or are thinking about it, I swear!