Do you have a lot of US pennies produced before 1982? Don't use them in a store-- the copper in one penny is worth almost two cents, and in a system where money is no longer backed by gold, it's a bastion of value in a sea of theoretical worth.
At the moment, about 15-20% of pennies in circulation are good pre-1982 copper, so for infinite money, buy lots of pennies from a bank, keep the copper ones, and exchange the rest for bills. Repeat until you have all copper, then sell it for scrap. Keep doing this and you'll make a mint.