Nice and hot(not roasting though) is what youre looking for. If its too cold or you pull too hard you risk stretching them.
Ive reused a few hope ones and when it comes off, its not as sticky so take care not to touch the sticky surface or youll impart grease from your fingers onto it.
As to reapplying some sort of adhesive :? possibly look towards putting it directly on the fork, reapplying the decals then wiping off any excess sticky. :?