Use heat sparingly. I have curly hair, so I straighten it often. I do not blow dry my hair before I straighten it so I can reduce damage. If you do have to use heat appliances, use the least amount of heat possible. Let your hair air dry as long as possible before you use a hair dryer. If you use a flat iron, use the lowest heat that will straighten your hair in one pass.
Before you use a blow dryer or straighten your hair, use a heat protection spray. These products coat your hair with an oil to protect your hair from heat damage, and they can also help repair damaged hair. I've used drug store brands, but they left my hair oily and limp after a few hours. I prefer to use CHI heat protectant because it just works better than cheaper brands.
I recently stopped using CHI and started making my own. I really prefer making my own heat protection spray because I think it protects my hair just as well, and it helps my hair get straighter. I can wear my hair straight for two full days before I have to wash it again when I use coconut oil, which means I straighten half as much as before.
When you wash your hair, make sure you thoroughly saturate your hair with water before you shampoo. Use shampoo just at the roots and first few inches of hair so you don't strip the natural oil off the ends of your hair. Look for a shampoo made just for damaged hair. Rinse with cool water to help seal the hair shaft.
Use an intensive conditioner daily to repair damaged hair. Once a week, use a hair mask like this coconut and peppermint hair mask or this avocado and hot oil hair mask. These masks naturally repair damaged hair, so you will have smoother and healthier hair without any chemicals or additives.
Get regular trims even if you are trying to grow your hair out to cut off the dead ends. This will help your hair look healthy, which means less frizz.