How can I out of harm`s way a substantial mirror ontop of my fireplace mantle? I've seen it in "designer shows" where on earth the mirror just...

I've seen it in "designer shows" where on earth the mirror just seems to lay against the wall on the mantle. I don't want to in truth hang it on the wall, I want it to lean against the wall but am not sure how to keep it from slipping on the mantle minus putting something in front of it to hold it in place. Any accepted wisdom?
Decorate walls!?
Answers:    If you want to lean the mirror but are afraid that it might slip then use a top mount and secure it to the wall.
Buy some line and attach it to each side of the back of the mirror and afterwards attach it to the wall, behind the mirror.
Make sure that you hit a stud.