Correct me if I'm wrong, and if I am, I'll eat a nipple.
In cultures where bare-breastedness is the norm, women (nor men) aren't particularly sexually sensitive about the nipples. If you don't feel any erotic sensation about your own nipples, do both of you a favor and say so; or experiment with pinch/tongue/nibble/rub/slow/rough/silk/etc. play until you find something you like. I would give my front teeth (well, maybe just the crown) for a lover to play with my nipples until they were sore. =)
Ice! How could I forget the pleasures of ice? (go easy, frostbite != erotic) Hot wax? That's a bit on the xtreme side, not for everyone.

I have several comments on this:

• To begin with, I think more women suffer from this problem than men – the world is full of breast men, and has but few breast women, and most couples are heterosexual.
• Second, male nipples can be erogenous – I know, being a male who's sensitive to nipple stimulation.
• Third, the nipples have lots of nerves both in people coming from bare-breasted and in people coming from cover breasted societies – though individuals of both sexes and all cultures might find that stimulated nipples irritate rather than please.