Split ends are forever, but once you've trimmed those off, you can get started re-growing and protecting the hair you have that's still in reasonably good shape. For supplements, it seems like just about every model swears by Viviscal, which is supposed to help your hair grow stronger, thicker, and more quickly.
You also can't go wrong with a good hair treatment—use something like Klorane Mask with Mango Butter to pack as much moisture as you can into your strands. It'll make your hair look smoother, give back some of its elasticity, and keep breakage to a minimum more generally. The easiest damage triage treatment we've run across is Alterna Caviar Anti-Aging Overnight Hair Rescue and this one by Rene Furterer. They call themselves masks—you literally just sleep in it and then wash it out in the morning.