The best way I have found to euthanize a fish is to use tricaine methanesulfonate (sometimes called Finquel, MS-222, or Tricaine) buffered with pH up or baking soda. Add the solution very slowly until the gills stop moving. After about 10 minutes add a shot of grain alcohol just to be sure. It causes no discernible discomfort when it's done correctly.
Be sure the fish isn't just bored or needs a water change before you euthanize it though.