I have the same exact problem!! I have naturally curly/wavy hair and I hate it! I straighten my hair almost every day. Even if it's humid outside my hair turns a little wavy because of the moisture in the air. But to keep this from happening, I use Tres Two Spray, by TRESemme. It works amazing! I spray my hair all over with it after I straighten my hair. Then my hair turns all hard so I brush through it and it makes it really soft. It's a very popular brand so you should be able to find it at any hair salon or beauty place. The bottle is a large airsoft bottle that is black but it has almost an olive green color on it. I hope this helps!