The Hoffman Technique
This exercise can be useful in helping to remove any impediments to your nipples protruding. You can perform the Hoffman technique by placing both of your thumbs at the base of your nipple and using a gentle motion to pull your thumbs away from each other. According to Mother and Child Health, you can repeat this exercise two times a day at the beginning, and slowly work up to five times daily. The website notes that this exercise can be a helpful way to prepare your breasts for breastfeeding, and can be done while in the latter stages of your pregnancy.
Discuss use of this technique with your Health Care Provider if you have a history of premature birth, or factors currently predisposing you to premature labor. I would not recommend starting this technique until the last trimester of pregnancy. The hormones of pregnancy often help to loosen adhesions without any interventions.
With flat or inverted nipples, it is particularly important to put your baby to your breast as soon after the birth as possible. Allowing your newborn access to your breast in the first hour or two has an imprinting effect.