Repetition can work wonders when you are trying to annoy someone with a prank. Place a sticker on your friend’s back, let them walk around with the sticker on for a while, and hopefully a few people will notice and laugh. Eventually they themselves will notice it or some generous soul will comment on their new accessory. Once they find the sticker and remove it, place another or the same sticker on them again in the same general location. After they find out or you tell them you are putting the stickers on them, they will be watching your every move. They will jerk at the slightest touch and be on the look out for you trying to affix any more stickers on their back.
You can now act like you are pressing something on their back or shoulder but in reality you will not be placing anything on them. They will look or reach around each time you touch them to feel if you have put something on them again. Watch your friends squirm and sometimes get really upset. After doing it about 5 or 6 times they won’t even bother to look when you touch them. That’s the time to actually reattach the sticker and start the whole process over again. Try this at the mall or at festivals where there are large crowds and your friends wouldn’t want to look like a dork by wearing a sticker on their back.