From BME Encyclopedia
Nipple stretching is a term for modifications which either increase the bulk of the nipple, lengthen it, or both. A few methods are in use:
By "abusing" your nipple with multiple temporary nipple piercings and play piercing, the bulk of the nipple can be increased. However, this size increase is due to internal scarring, and can result in a enlarged nipples which are lumpy and unsightly. In addition, this scarring can potentially block milk ducts, resulting in mastitis in rare cases.
Vacuum pumping of the nipples (see the entry on nipple pumping), either with a commercial pump, or with improvised devices such as syringes and snakebite kits, causes the nipple to become engorged and enlarged. This enlargement is temporary, but if done regularly, can become permanent as the tissues stretch.
Similar to pumping is nipple banding, where an elastrator band or other restrictive device is tied or placed around the base of the nipple. This both reshapes the nipple area, lifing the areola out and making it appear to be a part of the nipple itself. It also acts similarly to a "cock ring", partially constricting blood flow and thereby increasing the blood pressure inside the nipple and causing it to engorge — thus having similar results to pumping. It should be noted that tight elastration for an extended period can kill your nipple. If this happens, the necrotic tissue would have to be excised, resulting in nipple removal.
The nipple can also be lengthened by regularly "pulling on it". In general this is done by devices such as nipple trainers and nipple stretchers, which are custom jewelry designed to push the nipple out from behind. Weights attached to standard nipple piercings can also be used for stretching, but this method can easily result in migration of the piercings or even tearing.