fortunately enough i spend a lot of time thinking about this exact thing. i've got a foolproof plan for three wishes, but one wish makes it a bit harder. it's a fun thing to ponder and try to figure out how to get away with :]
"I wish that I had a wallet that always had exactly one authentic American $20 bill inside, no matter what; with this wish executed exactly as I intend it, and no further changes to the world as a result of this wish."
good luck ;p