This only works if you have a certain amount of comfort with this girl and a certain amount of sexual Tension built.

Walk up to her and offer your hand (like how you say hi to your friends) but when she grabs your hand move yours up and down for no more then 3 seconds, she will start doing the same. At this point you can make a number of awesome sexual tension building jokes and even call her out and say that she is awful and handjobs and from there that you need to teach her some technique. I recently did this to a girl and it's now our secret handshake, in class I told her she owed me a handjob and she looked at me and whispered later, so yea, it works, try it.