Ooooh Jett used to do this!!!  Man it was soo annoying.  He only started doing it around 8 months or so, and he hurt them so much I had to use nipple cream to heal them as they got so cracked and sore!  Yet I didn't need any cream when he was born!
 
Sorry, I don't have any advice, just letting you know my son did the same thing, but he did stop doing it after a while.  Although when he would do it, I would take him off the boob, put him down and sternly tell him off, it would hurt me soo bad!