I experience the same thing from time to time.

What kind of flat iron are you using, i.e., is it ceramic, titanium?

I used to have the same problem using my Royale, which is in the same boat as the GHI, the CHI, and others.

I switched to a titanium flat iron by Jilbere from Sally's Beauty Supply, and found it did a MUCH better job of straightening my hair (versus styling, which is what the others seem to do).

You may also have luck trying a Maxiglide (very expensive, kind of hard to find locally, but it's totally worth the splurge)

For technique, try ironing much smaller sections of hair in your problem area, and go slower down the hair shaft. That way, you can just be more effective with the heat setting you use rather than bumping it up.

As far as the ends flipping outward, unfortunately, that could come with the territory of your hair length. I've found that once your hair gets an inch or two past your shoulders, the length will start to weigh it down and hopefully it will be less prone to the upflip.

One question, do you blowdry? If so, your blowdry technique could also affect the end result of your ironing.

Lastly, you may want to consider getting some sort of heat protection or flat ironing spritz or spray (I know, I know, no specialty products). You could spend $4 on the Tresemme Heat Tamer spray, which I have had a lot of success with in the past, OR consider spending $8 on the Neutrogena Triple Moisture Silk Touch Creme. I've found that sometimes, putting a product will weigh your hair down and make it more conducive to the flat iron, as well as protecting from all the heat. Although, if you don't have split ends, what you're doing must be working pretty well!

I'm no expert in this stuff, and I'm no hair stylist, just a girl with curly hair who couldn't put the flat iron down to save my life! I hope something here is helpful :)