If there is a local LLL or breastfeeding clinic they will have lots of techniques to try. First they will determine if it is flat or inverted. Medela makes nipple shells and nipple shields. Both were helpful for me in the beginning to help my daughter latch on. I had very large breasts and very flat nipples with my first baby... and since she was only 5 lbs at birth her tiny mouth had a hard time opening wide enough to get all of what she needed to in her mouth without the nipple sheild. Breastfeeding was compounded by thrush and milk blisters and many other problems that caused severe pain (pain is not normal in breastfeeding ~ it shows there is something wrong). DD had a very difficult time latching on so she'd end up just somewhat licking what she could get until we began using the nipple sheild. I tried all sorts of things and finally when DD was 5 mo old it seemed to get much better. I even got her to latch on without the nipple sheild. Eventually DD stretched out the tissue of the nipples and was able to latch on without difficulty. ... oh and after she was weaned I went down 4 cup sizes from my PRE pregnancy size. For my second baby I had tiny breasts and very stretched out nipples so it was really only the first one that I had a lot of trouble getting to latch on due to flat nipples. My second had other breastfeeding issues due to prematurity.
For Both babies my local breastfeeding clinic was essential to getting me through. They provided support, encouragement and lots of hands on knowledge that got my babies to eat like they needed to.