Use a supplement called MSM (found at GNC), or use the vitamins called "Women's Hair, Skin and Nails Formula", also found at GNC for $10.

Other things that help your hair grow are leafy, dark green vegetables, drinking lots of water, and exercising. Minimizing harsh treatment and manipulation of the hair, such as perms, extensions, pulling/tension on the hair significantly helps. I've noticed tremendous improvement in the health and growth of my hair when I don't manipulate and chemically treat it.