To remove a stamp from an envelope, put the envelope in the freezer for a few hours. When you remove it, the stamp will pop right off. The adhesive is still good and the stamp can then be placed wherever you want to use it.
I read somewhere you could put a little paint thinner on a q-tip and rub it on the inside of the envelope. I didn't have any, so I tried nail polish remover. Oh my goodness--the stamp almost immediately peeled off clean, and it was still sticky! It will be so easy to reuse!
The finger nail polish suggestion worked great. They are not sticky, but glue sticks work great. The freezer trick nor the steam trick worked as well as the finger nail polish!