What DStubb said.

You should serve it above and below w/dental floss if it is riding up and down. You can also put a spot of super-glue on the juncture of the string serving/knot.

I don't want to offend you at all but like DStubb said, if tied correctly that shouldn't happen. Are you certain you are getting it tied right? I have tied hundreds and I still get one wrong now and then, it takes some thought to get it right. I also put a bar of some sort (plier handle) through mine and put my opposing hand on the string splitting the loop between my thumb and forefinger andPUSH/PULL with all my might....and I am a big boy! I have actually broken them but I promise one thing...it's tight.

Lastly, may I ask what type of bow you are shooting? Is it a hyperviolent one?