How to make hair stay straight?

Hi,
My hair is naturally wavy/curly, and somewhat thick. After showering, and washing and drying my hair, I have to straighten it to make it look tidy, otherwise it doesn't look like I've even brushed it (even though I have). So here's the problem:
I have both a straightening iron and a straightening brush, both straighten my hair fantastically well, they both smooth out all the curls, even my fringe (which is curlier as it's shorter). But I can spend ages straightening my hair until it's perfectly straight, but it never seems to STAY straight. Around afternoon, the waves/curls are coming back, even if I use heat protector.
Please help.
Thank you.
Animated Demonstration Video