What are my options for sagging nipples after breastfeeding?

I've nursed four children and each one of them would pull against the breast while latched on. It caused them to stretch significantly. With my first three, the nipples returned mostly back to normal, but after the last one they remained stretched out. I'm very young so I was wondering if it is possible that with time they will tighten up and, if not, how are stretched nipples fixed surgically?