I don't know how many of you already know how to do this, but for those of you who don't, it is very easy to remove stamps from envelopes. If it is an envelope you do not want to save, simply cut the stamp off of it, leaving about 1/4-1/8 inch (about 1/2 centimeter) of paper border, and place it with others in a bowl of water. If your envelopes are colored, place them in a different bowl than stamps with white envelopes, and keep an eye on them to make sure they don't bleed. The water should be cool to lukewarm, cooler for colored envelopes. Let them soak until the paper slips off the stamps. Self-stick stamps can take quite a while. Lay the stamps out on a cloth to dry. When dry, if they are not flat, you can press them in a heavy book, or under a pile of books.
If you have a postcard or an envelope you do not want to cut, simply cut a double layer of a coffee filter (I assume other absorbent paper would work well, though) that is slightly larger than the stamp, wet it, and place on top of the stamp. Cover with a glass, plate, etc to keep the water from evaporating. after about 10 - 30 minutes, the stamp should slide off. Note that I have never tried this with self-stick stamps, so I have no idea how long that would take.