When I am going to flat iron I usually wash and condition and after I shower I apply some type of serum, I like the Chi silk infusion. Then I let my hair air dry to cut down on some heat and having to use a blow dryer. If I want to cut down on the drying time I use a microfibre towel when I get out of the shower.
I then set my flat iron on the lowest setting possible to achieve straight hair, for me that is 350, if you don't have to use your flat iron on the hottest setting then don't.
Before I flat iron I section my hair and I spray on some heat protectant on each section.
I use a boar bristle brush and pull that through my hair as I'm pulling the flat iron down my hair. This brush tends to make my hair nice and soft.
Then if need be I finish off with a bit more serum.