Grease the toilet seats in all the restrooms using a product like Vaseline.
Or just use a tab of butter, Neosporin, Carmex, personal lubricant, even rubber cement, but usually the smell gives that one away. Just leave a fine film. Nobody will really notice until it is too late.
Make sure you do you business at home before you go in. After all, you don't want you little prank to backfire on you.
It's also the perfect trick for a house party.