Spray the glue with glass remover and wait for it to dry to a gummy consistency. If it's on the inside and you don't want to make a mess, put a paper towel over it to hold the cleaner on the glue. Repeat this a couple of times if it's a tough job. Then, take a razor blade and scrape. The glue should come right off. Ammonia breaks down most window glues so that it loses its adhesiveness.
To make this easier on yourself in the future...before removing the sticker, saturate it with window cleaner a few times and then scrape. It should come off pretty cleanly... :)