I have naturally straight hair, thanks to my Native American heritage and love it. I wear it all 1 length and straight.
I did, however, try the CHI Enviro treatment (no chemicals) my hair dresser offered for a more sleek, straight look. I paid about $150 ( special... Normally higher). She washed my hair with the special shampoo, leg it sit a few minutes, rinsed out. Then applied the product ( brushes on, covers each strand). Waited a few minutes. Dried my hair then flat ironed all of it. You can't wear a clip or band for about a week. Use the CHI system shampoo, conditioner and mask ( included with the $150)
I did this in October and my hair is still so soft and healthy. The conditioning is great. Like I said, I don't use a flat iron. I let my hair air dry while I dress, make up, etc and my dry time is less than 5 minutes and just run my brush through it.
The CHI flat irons are great but please do not use that daily. You'll ruin your hair.
Good luck!!