I like to buy old things and spruce them up a bit, so this is just my little idea I had to fix up a cheap mirror I got from a thrift store.
Find a cheap mirror. I bought this one at a goodwill for only $1. Be sure that it has a decent size frame to work with (all the more to decorate).
Using a hot glue gun, do a straight line, and little dots along the line (once you spray paint it, it'll look like it's welded together and has metal ribbits). If you want you can add more, I personally did it just were the frame is pieced together.
Cover the mirror. Make sure none mirror is showing at all (other wise you'll have grey spots on it).
Lay the mirror on a flat surface, and spray it evenly with the spray paint. I recommend doing 2 coats to make sure it's all covered. Leave the mirror covered, you'll be spraying the finishing spray on it later.
Cover the detail you made earlier with the glue gun, with black paint. If the frame has some design in the wood, it'd be a good idea to cover that as well (to give it an antiqued look). Only do 1 section at a time (look at the following step before repeating this stage).
After you have covered a section with black paint, carefully wipe it away, leaving some in the creases. Once you have done this, you may go and antique the other sections.
After you have finished antiquing it, lay it on a flat surface and spray it evenly with sealer. 2 coats is recommended.
Set aside your decorations. The little bug, and the cross are from old/broken jewelry I had, and the plastic gears came out of a cheap clock that didn't work anymore. The plastic gears I painted gold so they'd look better. Just about anything works.
With your handy dandy glue gun, attach your decorations to the mirror. Other than the the little decorations, I glued a chain along the top of the mirror.
The end result should be something like this :D