I don't straighten my hair often, because I have big curly (banana curls) that are stubborn and frizzy and thick and pretty much do what they want.
However, when I do straighten my hair there is always one product I always have on hand : Ion Anti- Frizz Solutions Heat Protectant Straightening Iron Spray.
Honestly, I swear by any Ion product ( or anything that Sally's sells) because they're professional beauty products and always do a better job than that crap you buy at the pharmacy.
You should always use a straightening spray when you take a flat iron to your hair because the heat can damage it. Use the Ion heat protectant because it really will keep your hair safe. It also makes straightening your hair easy, and I garuntee that when you're done, you'll notice that it eliminated all of your frizz and left it smoother than any other straightening spray you could ever buy.