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.
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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.