Eliza -

Try the fabric/hobby shops for similar label removers without the restriction on use on metals. There are several different kinds around, and a few of them actually sort of work.

An "office accessory" commonly used for such things is "Mentholatum" that has a penetrating oil/grease base and mades the stuff smell nice for a while. Just smear a little on the sticky and rub until it isn't sticky any more. It works great on "aged" masking tape that can't be removed by anything else short of a chisel or a grinder, neither of which is recommended for nicely finished stuff.

John