I might say, "You probably have more money than I do. How about you give me some money instead?" Just kidding - I probably wouldn't have the nerve.
Acutally, I'd probably say something like, "I'm really sorry, but, I don't like to give money away to complete strangers." (Thanks Parthon for that idea). "Sorry, I have hardly any money myself", I'd probably add if pressed. It's true.
I think the thing to avoid is seeming at all snobbish, cruel, lacking in empathy, etc.
Camouflaging yourself as someone poor might dissuade anyone from asking you for anything. If I lived somewhere with a lot of beggars, I might actually go out of my way to make myself look as poor as I am, because I really don't like having to say no to people.