Cut the envelope around the stamp and then apply some liquid (water, adhesive remover or other) on the back of the envelope where the stamp is attached. let it soak for several seconds to make the adhesive dissolve a little and you will be able do remove the stamp easier without damaging it.
As liquid for soaking the paper some people use:
- Adhesive remover - Heptane
- Water - the simplest solution. Warm water works better
- A pen with lubricant, for example "WD-40 no mess pen"
If you don't mind getting the stamp wet, then simply put it in water for several seconds and the paper envelope will separate from the stamp very easily. After that let the stamp dry and you are ready. You can dry it by putting it on kitchen paper with the sticky part upwards - to avoid it sticking to the paper.
You can also put the stamp on transparent plastic film and let it dry on that. It will stick on it and you can cut the plastic film next to the stamp edges. Thus way you will not have problems with the sticky back of the stamp and can examine the front and the back of the stamp easily.
Another way to remove its stickiness is to put it in talc which will cover the back of the stamp.
Actually you can't use stamps for second time as they apply ink on it in the post office.