Have you used a serum before? Serums seal the hair and prevent moisture from getting to the hair.
When I pressed my hair, I deep conditioned before pressing and got most of my moisture there. I also did a wet-to-dry press, which some say helps with moisture, only I used a heat protectant instead of water as I used to do in the past. I then went over with the flat iron one more time after applying a serum to seal. My hair didn't revert until I washed it. I only wore the press for 3 days coz I needed to braid it while I had a long weekend off, and I like to braid shrunken hair, but it wasn't looking like it had plans to revert any time soon.
My hair in braids ready for washing, 3 days after the press: