Stickers can leave behind a nasty residue on glass and mirrors. A razor blade is the best tool to remove the sticker effectively. Any leftover residue can be cleaned up with rubbing alcohol.
Wet the mirror and stickers with warm, soapy water.
Scrape the sticker off of the mirror with a razor blade. For safety, use a razor blade that includes a holder. This item is available at hardware stores.
Apply rubbing alcohol to a cloth or cotton ball and rub over any excess sticker residue. Wipe with a dry cloth. If adhesive residue remains, try a adhesive-removal product such as Goo-Gone.