I asked this because while sorting my Australian kiloware I figured that new Australian stamps were very hard to take off paper. The old stamps would come off paper after just a few minutes of soaking in water, but these new stamps just wouldn’t budge, refusing to peel off paper easily. In Hindi, we would call them ziddi or dheet (stubborn). It seems many modern stamps, especially from USA, UK, Australia and few other countries are “self-adhesive” stamps, like stickers. I am not surprised that philatelists don’t like these stamps.
Most collectors recommended soaking stamps in lukewarm water and taking them out after some time. Dry them on blotting paper or an old newspaper, and then putting them in or under a heavy book to flatten them completely. It is useful to change water after a few soaks if you have many stamps to soak.
It is best to segregate stamps of colored paper and soak them separately. Colored paper sometimes runs color and is likely to spoil other stamps in the water.
If you don’t want to soak your stamps, you could also use a sponge. Wet the sponge thoroughly and leave your stamp on it, paper side.
Some suggested using a little talc powder to help reduce the stickiness after you remove the paper from self adhesive stamps. However this is debatable as many other collectors indicated that talc is more likely to ruin the stamps than help them.
All the above are good suggestions for most stamps that are not “self-adhesive” stamps. But what about self-adhesive stamps? It seems these require special material to remove from paper. Few videos below help you with methods of removing these self-adhesive stamps. I haven’t tried these methods. However what I have discovered that for self-adhesive stamps, you need to soak them in water for a much longer time. I have managed to peel these stamps off paper after leaving them in water for three to four hours. Even then, there is a possibility that you will damage the stamps, so you have to be very careful. Also after peeling off the paper, some glue may still remain on the stamp. Simply dip them in water again and wash off the glue gently with your thumb while the stamps is in water.
It is best to try to peel off self-adhesive stamps from paper only if you have duplicates. Else it is best to just leave them on paper with the postmark intact.