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.