Straightening thick hair?

I straighten my hair quite often. I have very thick and poufy hair. When i get done the back is always just puffed out like none other. How can i get it to look like its straight and thin (without getting a brazilian blowout)? I use like a revlon straightener i suppose. It goes up to 25 and i put it up to 25 but my hair just looks not absolutely straight. Thanks i appreciate it!!