Do you have any $2 bills in your wallet?  You might want to start carrying those around.  Althought they are not really all that rare, people tend to think they are.  In a recent experiment, those who had $2 bills were less likely to let go of them or at least save them for a special occasion.  If you are giving money as a gift, consider it in $2 bills there also.  $50 or $100 bills will likely be held on to by the recepient a lot longer.  Most people think twice before breaking a large or unusual bill or coin.