Heat damage in general will do a couple things to your hair, it will damage it, change the texture of it and prevent future growth. Blow dryers, curling and flat irons are great tools to use but not meant to be used everyday!
Using a deep conditioner at least once a week, treat ends of your hair with coconut oil at night before going to sleep, allowing your hair to air dry sometimes and using products to protect your hair from heat damage are some things you can do to protect your hair. And finding the right products that work for you is the challenge sometimes, but worth it when you find them.
Follow the directions and do test first. I know it is time consuming and inconvenient but the instructions give suggested times for a reason. Find the shortest amount of time you need to leave the straighener on and still get the results you desire. When you say straightener, i assume you mean relaxer. The timing starts from when you apply the first amount of relaxer to your hair so choose carefully which section of hair you want to have the relaxer on the longest.
A heat protection serum is a must...Fekkia hair products are pretty good for a drugstore brand but why don't you stop by a high end hair salon in your area and ask one of the stylist what they reccomend. A good stylist can take one look at your hair and tell you exactly what brand would work for you...believe me, I've done it before...stylist are more than happy to show off thier expertise! :-))