Biotin works for me. Also trim your ends at least every 6 weeks to keep split ends away. Once the ends of the hair start breaking off, (you'll notice more loss of hair in the shower or there's more hair on your brush now), it starts moving up the hair shaft and keeps breaking, stopping your hair from achieving length.
Also make sure you drink plenty of liquids and eat a well balanced diet with lots of foods in B12, Iron and Omega, such as:
Also make sure your hair is always hydrated, so use sulfate free conditioners, and always use an after leave-in conditioner, applying more to the ends. Stop wearing pony tails if you can help it and lighten up on the HOT air when blow drying. Anything that will dry your hair, will break your hair, causing damage, that won't help it grow.
Finally, if you color your hair, use semi-permanents instead of permanents, if you can help it. If you have to use permanent with ammonia, invest in conditioners and leave-ins.