While receiving a blow job, just after the point of ejaculation, quickly pull your penis out of her mouth, and swing your body so that your penis hits her like a bat. If she spits out your cum, she must give you two pieces of candy. If she bleeds, she must buy you a large bag of candy.
If you fail to do one of these things, you may try again or let a friend take a shot.
Oral Piñatas are my favorite, because I get a blowjob and candy!