My hair gets curly too fast!?

I'm a guy and when I go out, I usually straighten my hair using a hair iron. After that, I put a bit of hairspray on it, just to keep it where it should be. It stays the way it should be (straight) for about 2 hours or so, and then just makes a weird horizontal wave on the left side of my hair. I tried various products in various amounts, nothing helped it. What do you suggest me to do to make it straight longer?