I've received so much help, advice and enjoyment from this site that I thought it was my turn to try and help out a bit and make a little tutorial showing how I remove the modern US self adhesive stamps from envelopes without damaging them. Hope this helps someone out.
Here goes nothing:
I wish I knew how many used self adhesive stamps I've ruined in the past by soaking them in water, carefully trying to peel them from the paper and then finishing them off by using my thumb to try and push all the excess gum off the back of the stamp. Many times, I ended up with a thin piece of paper that rolled itself up into a cone and refused to stay flat. Other times, remaining gum would stick to another stamp and ruin that one as well. Very frustrated, I just gave up.
With some help from here and a little practice, I've been using 2 items to flawlessly (for the most part) remove these used self adhesive stamps from the envelopes. Here's how I now do it:
Tools:
1 can of Pure Citrus air freshener purchased at Home Depot
1 small bottle of Johnson and Johnson baby powder.

The subjects:

First thing to do is lay out a piece of paper towel to absorb the over spray.
Next lay the stamps face down on the paper towel - This is important as too much wetness on the face of the stamp can cause the newer spray type cancels to run. On the other hand, red cancels actually don't run at all like they do when soaking in water.

Now, lightly spray the back of each of them. Just enough so they are covered. You'll be able to see through the envelope at this point and see the stamp outline. When I'm doing this for myself, I'll usually spray 10-15 stamps at a time.

Wait about 30-45 seconds or so,if possible, lift up the stamp and just kind of peel the paper from the stamp. Not the stamp from the paper. Doing it this way helps minimize putting a bend in the stamp on the corner. Try not to handle the front of the stamp on the cancel as you may smudge it if it's too wet. Once you get it pulled back a few millimeters, you can just use your fingers to grab enough of the stamp to prevent creasing it. It will easily lift right off the envelope.

Next, if the stamp curled a bit, it's not a bad idea to curl it back the opposite way to try and get them as flat as possible. Set the stamps off to the side, face down. You'll notice that you can see a white square or rectangle in the center of the stamp while it's still damp. When the stamp dries enough, the white square will fade away.
Once the stamp looks like a normal stamp on the back again, it's ready for the baby powder. This usually takes 30 minutes or so.
I use a little tray to dump a little powder in a time - yeah I know what that looks like :)


Then, just dip the back of the stamp in the pile so just a little gets on the back of stamp. Use your fingers to spread it around the entire back, give it a couple of flicks to get the excess powder off and you're ready to put your stamps in your album.

And there ya have it