Those with curly hair who straighten regularly

Mine is really fine and wavy/frizzy now that I'm older. It looks better straight to me though. I do most of that with a blow dryer and then run a flat iron on the ones that frame my face. I use this shampoo and conditioner: Biolage Fiberstrong. A leave-in conditioning spray and some styling product (I alternate between mousse, a light weight gel, thickening agent...just depends) and finally finish off with CHI oil for the ends of my hair.

Any type of thermal spray will work to protect it.

I do a few things to minimize heat damage on my curly hair because I prefer it straight. Washing it every three days, with dry shampoo in between helps. Usually by day 3 I wear it 1/2 up or in a pony.

On dry shampoo days, I will use blow dryer to straighten any fly always near my face using a large round brush or paddle brush. The heat from the dryer is less damaging than a flat iron. I do the same with the area around my neck because it sometimes curls or frizzes there overnight. From sleeping I sometimes get a back part. Dry shampoo or a root lifting spray plus blow drying it the opposite way of the part fixes this without needing to wet or wash it.

On wash days, product makes a huge difference in how long it will stay straight and not get frizzy/curly. I use a primer, some oil on the ends, and an all over styling cream. Then I dry it with my fingers till it's about 1/2 dry. Then I section it off and dry it using a large round brush. My hairstylist suggested switching to a wooden brush and I was skeptical, but it makes a huge difference in the smoothness. I finish with a shine spray and sometimes, hairspray.

I rarely use my flat iron since I started taking more time with the blow dryer. I think eliminating the flat iron and only washing every 2-3 days are the best ways to avoid damage as much as possible.

My routine is very similar to judy.  I have thick, curly hair that is colored so that makes the dryness even worse.  Shampoo no more than twice a week with a good quality, moisturizing shampoo/conditioner.  I'm using Pureology right now.  The system works so well I don't even have a flatiron anymore (DD moved out with it 3 years ago and I never replaced it).

For drying, I recommend investing in a good ionic hairdryer.  My preference is Elchim, though T3 is good as well.  Before I dry, I use a straightening/styling cream.  This one by Kiehl's does a really good job for me, both for straightening and controlling the humidity frizz.  I live in Houston, so that's a pretty good test of how well it works!  Get a good round brush with natural bristles.  The bigger round it is, the straighter it will be.  I keep a small one on hand for days I want a bit of curl on the ends.  I dry in sections and start with the underneath parts first.   If I want  a bit more volume, I will put a few jumbo hot rollers in.  It helps smooth it out and add a bit of poof at the roots.  I always finish off with a bit of Kiehl's Styling Serum or some moroccan oil.