how to deepthroat?

My boyfriends dick is 6 inches and I can only take 3 or 4 inches in because my mouth is really small. Any tips on how to safely deepthroat without puking? I heard that squeezing your left thumb works, but I don't think I would want to try it until I am for sure that it does. Should I be in certain positions to deepthroat? His penis is not curved or anything