Pure orange oil (not to be confused with an orange-based cleaner) is excellent to remove the gummy residue left after removing a label. Do a try before if you're unsure about the plastic of the holder, but on most plastics, orange oil is harmless. Alternatively, lighter gas can do the same job (with the same caution about the plastic).
You can get pure orange oil in a natural products shop (but it's costly in small quantities).