Does your flat iron have temperature control? I have baby fine hair, and have had decent luck using a Sedu Revolution at the lowest heat setting.
I've tried some different heat protectants, and so far the only one I've been really impressed with has been Aveda Brilliant __something__, but I'm not wild about the smell after ironing.
Curly Hair Solutions ReMane Straight seemed to protect my hair well, but I was able to wet it and have my curls return. I haven't used it in humid weather, so I don't know from personal experience how well it works then. I'm afraid I tried blowfrying first, and that was a total fiasco for me, as I don't have a dryer with a wind tunnel effect. As I so rarely feel a desire to wear my hair straight, I haven't had occasion to try it again yet, but will, as I've generally had very good luck with their products.
Baby Fine 3B, normal density and elasticity, low porosity
CGing since July 2008