A couple ways, one is soaking in water that is close to body temp, 2nd is using a 100% non-aerosal Pure Citrus air freshener (CVS carries), spray the back let set for a minute or so, peel the paper easly from the stamp then rub baby powder on the back because most of the glue will still be on the stamp. For bot ways put between a couple paper towels and stack something heavy on the overnight to get them good and flat.
Additional note about soaking, I stay away from red envelope and red cancelations, they will get the stamp reddish color. uUse the air freshener. Trail and error, try on common invaluable stamps first.
Great tip grr0003! i get a lot of mail from abroad and keep the stamps but never quite know how to remove them from the envelope, so I've started to just kind of tear widely around them. Going to try the Pure Citrus suggestion...