Use a protectorant and/or a conditioner every time before you straighten. Make sure your hair is DRY before you apply heat. Go as long as you can without washing so the heat style will stay in. This will minimize reapplications of heat.

I had an aunt with gorgeous, beautiful, long, black hair. It looked dead after years of her applying heat.

Also helpful, get a brand of styler that is supposed to help your hair get healthier and won't damage or burn it. My aunt got Nume, which my hair is much happier to let me apply every 7-10 days without curling up all kinky and unhappy.

I straighten 1-2 days after washing now, ever since I got it cut.