I like to mix a little butter (avocado, hemp, macadamia nut...) with an equal amount of serum (I like Chi silk infusion and Redken's heat glide). I mix them up until they are completely combined and then apply them to wet, freshly washed and conditioned hair right after I use my leave in.
Then I comb them through and band my hair in two ponytails. I let my hair air dry like this all day. When I take the bands out my hair is almost as straight as if I had ironed it. The trick is making sure the hair is completely dry before taking it out.