I used to work in a big named beauty store which sold thousands of flat irons a year. I learned many things about them. These are the main rules I was taught:
1) Don't buy the cheaper ones. They do not straighten the hair well and they c*$p out after a very short time. Spend the money and buy the professional ones like Chi or Blue Saphire. I have the Saphire one and it is great! Trust me, I have hair that will not cooperate and it straightens easily and quickly with a professional iron. Stay away from Hot Tools, even the ceramic ones, they are not as good as they used to be. My store would get about 20 returns a week of broken Hot Tools flat irons
2) You should only straighten your hair about twice a week. Otherwise you are really damaging it. I have seen many girls come into the store with dried out, damaged hair because they would use a flat iron everyday.
3) Use a heat protectant product before hand. I have Paul Mitchells Seal and Shine which is nice. I also have a product by Wella but I cannnot remember the name of it off hand. Both are used before hand and help to protect the hair from the high heat. Also they are a great buy because they can be used before using a blow dryer or a curling iron, etc.
4) NEVER EVER EVER use a flat iron on wet hair. I don't care what Revlon or Loreal say, those flat irons for wet/dry hair are the worst things ever.