Pour 1 cup of warm water into a bowl. Add 1 teaspoon of dishwashing detergent and stir the two ingredients to make a soapy solution.
Wet a clean cloth with the diluted detergent. Wring out excess moisture. Lay the cloth over the price tag and leave it there for 10 minutes.
Remove the damp cloth from the surface of the mirror. Position the edge of a credit card next to the edge of the sticker and gently scrape up the softened price tag. Repeat this to remove as much of the sticker paper as possible.
Loosen the grip of the price tag sticker adhesive by smearing cooking oil over the glue. Apply a thin coating of oil to the adhesive and allow it to sit for one to two minutes.
Scrub the mirror with the soapy cloth to remove the price tag adhesive. Dry the surface with another cloth.
Spray the surface of your mirror with window cleaner. Wipe the mirror with a dry cloth to remove oily residue and any last traces of glue.