I haven't heat styled in years, but my mother used to heat straighten my hair weekly all through childhood. Never had a problem with length retention or curls reverting properly. Her preferred method was heat protectant, stretching everything in a roller set under a hooded dryer until dry, then finishing with a low heat round-brush blowout for shine and volume.

There are other methods as well, but I think it's still good practice not to use your full-heat styling tool on wet hair. Letting it dry mostly stretched, then finishing with a lower heat setting is gentler.

My hair got heat damaged last year when I went to a salon and she ran the hot flat iron through about 6-7 times (I told her I was afraid of heat damage as well). She was trying to make my hair shiny, but my natural hair color is a medium brown color, so it will never have a shiny look to it. 

The best way is to make sure you blow dry your hair as straight as possible and only do one pass with the flat iron.

Curls are extremely sensitive even to many tight wet pony tails can change/damage your curl pattern.  Any hear will likely damage your hair

what’s the best way to protect my natural hair from heat damage? my hair has been 100% natural (including protective styles) with no heat for the past 2.5 years and I wanted to do a silk press but am worried that it’ll mess up my natural curls. I know that straightening once shouldn’t make a difference but I’ve heard to not even risk it

question about hair straightening