How do you get paint off your mirror?

If you are removing drips, it is fairly straight forward. Most hardware stores carry a little something called a paint scraper. It is a metal (or plastic) handle with a single edged razor in it. Gently slide the blade (at a flat angle) along the surface of the glass of the mirror and scrape of the drips.
If you have completely covered the mirror with chalkboard paint, my suggestion is to buy another mirror. The time, effort, and frustration of removing that much paint is not worth what it will cost for the mirror.