Start with clean, detangled hair:
1) use some sort of heat protectant (serum, spray, etc.). I like the ones by Keracare. I also like to use a little oil. I use Natural Oasis.
2) make sure its not too hot (check to see if it singes a towel before it touches your hair),
3) but not too cool either; you shouldn't have to go over the section three or four times,
4) take small parts, especially if your hair is thick. if you rush through taking larger sections of hair, it won't come out as well
5) move slower at the roots and faster the further you go down to the ends. the idea is that you can focus more heat on the stronger new growth portion of your hair but a lot less on the older and weaker ends.
6) you can definately add curl; for more of a spiral, twist your hair around the flat iron; for a bump, slide the iron almost to the end, then turn it in the direction you want the hair to curl and finish sliding the hair through.
7) don't flat iron too often (I go with once per week or less) and never flat iron dirty hair
Also recommend a flat iron with temperature control (not just lo and high). If I am flat ironing my roots only, the setting can be higher. If I am touching up the ends only, its a lot lower. I have gotten pretty good with my ceramic iron, I can make several inches of new growth look freshly relaxed!
Ok, I just purchased a flat iron today, FINALLY!!! The one I brought was the 
