Excessive amounts of heat; not good for hair. I can literally count the number of times I have straightened my hair in my life. I am not joking. That sizzling sound you hear?? It's your hair burning. If you must straighten your hair, limit it to perhaps every time you get your hair steamed which should be every 2 weeks if possible.
I wash my hair once a week. That's not for everyone, but it works for me. I blow dry my hair then. It's not always possible but if you can try to use medium heat rather than high heat. Blow drying can be a good alternative to straightening; if you're in the salon ask the stylist to blow dry your hair with a round brush. The results are better than straightening: your hair is smooth, silky, shiny, and less flat and greasy.
This sounds weird but sunlight can be good for the hair. Spend 20 minutes outside in the sunshine. It's not always possible but when you can, do it! I am convinced my hair growth increases in the summertime. I can't back this up scientifically, but I am convinced. Try it.