Orange Pure Citrus air freshener spray.
However, the gum remains on the back of the stamp after it dries, so you have to use unscented baby powder, or scrape the gum off the back...
Using powder is messy, and scraping increases the possibility of damage to the stamp.
The biggest problem with Pure Citrus is that it can fade or blur postmarks.
There are numerous other chemicals that you can use.
I've given up on it, and now just trim close to the perforations and mount with the paper attached.
Another method is to avoid unsoakable self adhesives altogether.