How do I keep my hair pin straight all day?

Hi, I have thin, wavy/straight hair. But the very ends of my hair are always curled some way. When i straighten it, it starts out pin straight but 20 minutes later the ends begin to curl again. Is there any products or ways i can keep it from curling? I use Aussie heat protectant, my straightener is the Remington Wet to Straight 1 inch flat iron and the hairspray i use is the Got2B smooth operator. Any ideas?