If it's rubber-cement based, GoofOff will smear it around more than remove it, as reported above. Using the least aggressive method first, I'd say to try 91 percent or higher alcohol. Saturate the adhesive and let it work for a few minutes (a saturated rag pressed up against it works well). You can feel when it's dissolved by rubbing it with your finger, then just wipe it off. I learned about this when I used to do vinyl graphics where alcohol was the solvent of choice. I just used this method last week to replace the annual stickers on 5 cars.