How can I flat iron my hair with out it coming out being to greasy or to dry?

Each time I flat iron my hair turns out dry. One of my friends have oiled and straight hair.I asked her did she get a relaxer in her hair and she said no. She only flat irons it. And I asked her what product she use and she said she used nothing. So is there any method to get oiled (but not to greasy hair) straight hair.