How to protect damaged hair from flat irons?

My hair was naturally curly. But it's gotten a little less curly with age, plus it's gotten so long (a few inches past my boobs) that it doesn't really curl anymore. If I scrunch it, it waves. If I let it dry naturally, it's somewhere between wavy and straight, but with like a layer of frizz over it. I straighten it to get it completely straight and mostly to get rid of the frizz. I know it damages my hair, I know I can't STOP it, but what's a good product that would help?