Use a good shampoo and conditioner (leave it in for a few mins) and let your hair air dry instead of blow drying. You don't want to overdose your hair on heat.

Always use a heat protectant and a good quality flat iron, preferably with pure ceramic and tourmaline plates (I use the Karmin G3 salon pro styling iron, best iron around) and has temperature settings. Straighten slowly and steadily and use small sections to get it done faster and better.