Jane, You can use any of the products that remove adhesives(goo be gone) stuff or sometimes acetone works if your careful but reguardless of what remover you use it really only makes the glue soft an you'll need to scrap it with a razerblade scraper,it's still a little work.
Jane: As mentioned, acetone is excellent. BUT, do not allow any acetone to touch plastic. It will deform plastic! This can be purchased at a paint
or hardware store, i.e. Home Depot, Lowes, etc. Also, NAPA Auto Parts sells a liquid adhesive remover in a can. Works great. Another alternative is cigarette lighter fluid. Wal-Mart still sells it at their cigarette counter.