I'm looking to hang a mirror on the wall on my bathroom. It weighs about 20 or so lbs. There are no built in hooks on the back of the mirror? What should I use to hang this thing?
At Home Depot (or Lowes) you should be able to find these Spring Loaded Mirror Mounting Clips made specially for hanging a mirror. They are no where near as big as a j channel and the top two have a small spring inside of them so they kind of grasp the mirror (and snugly fit on the glass).
You put the two without the springs in the bottom and the mirror just sits in them. Two more (with the springs) added to the top prevent the mirror from falling forward. Of course it would be best if you can get them into a beam in your wall. And you will barely notice the clips on the mirror once it is in place.
Liquid Nails Mirror Adhesive, but it does say you need to use a mechanical support as well, such as a shelf angle or "J" channel.
Another option is Heavy Duty Construction Adhesive which will hold about anything. In our new house, we installed large, frameless mirrors in each bathroom using this and it worked great.
As Alex pointed out in the comments, the site for Heavy Duty Construction Adhesive does not recommend using it for mirrors. They do have two other products that are specifically for mirrors, Mirror Adhesive (LN-930) and Mirror Adhesive (LOW-VOC) (LN-730)
It didn't come with anything? The one 'frameless' mirror that I have is in the dining room, and it's probably 30 lbs, but it has a wooden frame behind it and a set of small metal clips that go over the edge onto the front of the mirror and then screw into the wooden frame. It really doesn't change the look, but it supports the whole thing very nicely.
I'd try to do something similar with another frameless mirror.