I use
Pure Citrus Air Freshener.
Spray it on the envelope side of the stamp, wait 2-3 seconds, and peel off the stamp. Let stamp dry, then rub a little baby powder on the sticky side.
Some stubborn stamps (or perhaps it is the envelope material) require a little longer wait before peeling.
This does not work with the old-fashioned water activated stamps, but works like a charm with self-adhesives. No thinning. Doesn't discolor stamps. Doesn't affect cancelations (with rare exception of some spray on cancels that seem to "run" if they get realy wet with the spray, so spray lightly on back side and peel before it soaks in too much).
If you're doing a lot of stamps, do it in a well-ventilated area unless you really like the smell of orange air freshener. I have the best smelling garage in the neighborhood.