I would suggest using a hair dresser oil, heat protection, and deep conditioning creams to protect your hair from and heat damage, repair and split ends, and keep it healthy. In all reality, keeping your hair healthy and getting trims are the best way to help it grow. Wearing your hair up at night also helps. Don't straighten, curl, or blow dry it much because the heat is very bad for it. When you go in for a trim, ask your hair dresser to just "dust" the ends so that you get rid of any split ends, but don't lose much length.
There's a theory that the horse shampoo 'Mane and Tail' grows hair more than 3 inches faster and longer than without by simply washing your hair with it every other night. Other than that, taking good care, maintaining, and patience will do the trick.