When I used to straighten, I found that I had better results when I used a clarifying/chelating shampoo first (I used Elucence or ORS Aloe). The d/c seemed to penetrate better and really moisturize my hair. I applied this with a quick steam treatment. I dont have a steamer, just a bonnet dryer.
The other thing was using a high enough temp. I used Sabino so I would flat iron on 400-425 because if I didn't, it would start poofing before I was done.