Having recently spent several weeks working on my portfolio, and accidentally ruining a day's worth of work in the flattening process, here is what I now do: heat the dry mount press; place the print face down on a sheet of acid free foam core, with a sheet of release paper on top; heat the print for a minute or two; place print into a sandwich of release paper under a 3/4' piece of MDF to cool. This gets it 90% flat. For 100%, I leave the print in the press, turn it off and let it cool. It comes out flat as a floor.
If you heat the print with the glossy side in contact with the release paper, you will often get a dimple, or the release paper will slightly emboss the glossy finish and ruin it. I was very surprised to find that the foam core does not ruin the coating.