Have used the Super Safe Stamp Lift Fluid laswabbie has linked to, on occasion, and it does work though it is not perfect. Seems to leave a bit of a stain along the edge of some stamps, but not all. It's to be applied to the backing paper, such as an old album page, so don't know how well it will penetrate the cardboard your stamps are on. Have never tried applying it to a stamp's face.
Whether or not I use it depends on the stamps. Sometimes it seems better to just soak the stamps and go with no gum copies, especially if I don't intend to use them in my main collection.