On top of what Idza suggested, you can also try these two methods (making a "breast sandwich" or doing a "nipple nudge"; taken from: http://www.bellybelly.com.au/breastfeeding/breastfeeding-with-inverted-or-flat-nipples/)
Breast sandwich: place your fingers an inch or so from the nipple, then compress the areola to flatten the breast. This creates something firm for baby to latch onto.
Or, nipple nudge: use the index finger of the hand that’s supporting your breast to push up and out from behind the nipple to make it more prominent.