The way may be your mirror, largely depends on your taste, skill and available time. Unless you entrust this work some craftsmen and on transformation of conventional mirrors in unusual.
For a round mirror, you can try to crochet openwork circle of thread of your favorite color. Plant on the edge of a thin rubber band and put it on the mirror. For square mirror weave beautiful ornament of macrame thread and stick it with a rubber adhesive on the back side.
In closing, you can mirror inlaid mosaic one of the surfaces of the beads, beads, colorful pieces of broken glass, securing them in a special solution.
If you like a miniature painting, sign the reverse side of your mirror. For a better safety cover painted with varnish. You can paint it something avant-garde, if it is in your nature.
If you want to change the image, make a mirror on the back pocket of a kind of transparent film to secure it along the contour of strong glue and leaving enough space to change the pictures. Similarly, you can insert pictures and change, and then your mirror is already half medallion.
You can make the semi-leather cover, decorating it with applique from different pieces of leather, and secure the back of the mirror rubber adhesive that did not need to be removed every time the mirror out of the case.
If your mirror is round, and you are still quite young and love the series about Smesharikov, there is no better model for the reverse side of your mirror than Nyusha! Also you can choose someone else from this merry company.
Well suited to decorate the back side of your mirror any image of something round: black and white yin-and-yang, images of ancient coins, buttons, clock face, etc.