Joel Spolsky: Thanks or No Thanks
My best friend came up with a system that saves the company he worked for around $100,000 a year.  His boss took notice and gave him two free tickets to a movie theater.  He quit that afternoon.  He told me a sincere thank you would have been ok.  He was getting paid to work there already after all.  The token bonus was taken as an insult, as if that was the value of his contribution.  If you give a bonus make sure it matches the value of the action.  If you can't measure the contribution in dollars, find some reward that isn't in dollars.