I saw a guy at a gun show selling holsters. He wet fit them on the spot. He used a food saver vacuum sealer to draw all the air out of the holster/gun combination, helping to do the initial fit. Then he used a rounded tool to press the leather into place where it needed extra help, like where there were indentations in the frame of the pistol. I don't remember whether he wrapped the gun in plastic first, but I suspect he did.
He also had some instructions to the buyers on how long to leave the gun in before pulling it out, but it was just a quick thing I saw, so I don't remember all of the details. The food saver seemed to do a good job though.
Brian.