Take a tab of butter from the lunchroom and grease the toilet seats. Just leave a fine film. Nobody will really notice until it is too late.

If you do not have an individually wrapped tab of butter, you can use Vaseline, Carmex, personal lubricant, even rubber cement, but usually the smell gives that one away.

Make sure you do your business before you go in. You don't want your little prank to backfire on you.