This one takes some effort...
1. Find person to f*ck with
2. Find out what kind of toothbursh he has.
3. Buy same toothbrush and use it for couple weeks
4. Take pictures of toothbursh sticking out of various locations, i.e. the dick hole in your underwear, sticking up from your a$$-neck...make sure you can identify the nomenclature of the toothbrush so he really thinks it's his.
5. Print pics, place pics in his shaving kit when he's not around.
6. Wait for reaction... it's hilarious.