That's exactly what I use, BTZ Turn up the Heat or Keracare Silken Seal (which I also love)
I've also used Ion's Heat Protectant (Sally's), it was also pretty good.
ETA: Silken Seal can be used while your hair is damp/wet, that way you can flat iron w/o applying anything else. Just make sure you distribute it as much as poosible to get all the hairs.