Actually there are PLENTY of women here who use heat regularly who retain their length just fine. It's all apples and oranges really and everyone's hair is different. Some can use it some can't, some can relax, some can't. Some can brush and comb, others can't. It's a huge generalization to say that those who use heat don't retain most of their length, because that's just not true. Since living out here in LA, I have seen SO MANY AMAZING HEAT STYLED HEADS it's ridiculous. I'm talking APL, BSB, MBL, and WL natural and relaxed heads who are using heat weekly to bi weekly... (Ok enough of my super PMS diatribe).
Basically as stated above, you've got to make sure you MOISTURIZE before using any heat. Deep condition 1-2x a week, moisturize at night and either pin curl/wrap/cross wrap your hair to maintain the straight style (which ever method works best for YOU). It's also really to your benefit to know what temperature will work best for YOUR hair that will get it straight enough without burning or damaging it. For me, I know that I will never use my flat iron higher than the 2nd setting of 284 degrees. Not unless I'm looking for trouble, lol. I flat iron in smaller sections, 2 quick passes, and done.
I think it's also best to invest in a good flat iron to avoid damage. I purchased mine for $60 at the local BSS and it is similar to a CHI. Although I'm looking at getting the Runway because I'm taken by the hype quite honestly. But really it wasn't until I got a good iron that my hair thrived and I've seen no damage. And as mentioned so many times before, do not flat iron if you hair isn't clean, and if you feel the need to "touch it up" you may as well just wash it again and start over.
I have tried the "good things" like minimal to no heat, braid outs, wash and go, low manipulation, and honestly it was those things that made me have to trim more often. I find when I use heat my hair actually does a lot better, maybe because it's used to it, or maybe for me flat ironed hair IS less manipulation.