Roger is the one that got it right. Use an Atomizer with a bit of glyceryn or photo-flo. Spray the back not the emulsion side. Put the prints under weights for a week and repeat the process.
If this is something that keeps coming up for you as it does for us, go to your nearest commercial glass installer and have them cut up some scraps into 8x10, 11x14 etc. They always have extra pieces of heavy plate glass from commercial installations and will cut it to size for you. Glass makes great weights --just use it in conjunction with a blotter pack. We have them in all sizes up to a 1 foot wide by 8 ft long. The long panoramas from the turn of the century always show up here rolled and brittle after being in a attic for a century and it often takes six months or more to flatten them doing it the way suggested.
Other suggestions to plasticize the originals by soaking will work if you are in a hurry and willing to pay the consequences of potential damage from loss of the emulsion near any cracks. It's doable, in fact you can even float the emulsion off of the backing and put it on a new substrate but this is not something to try on a client order until you've successfully done it with some of your own.
Timemark Photo Conservators