Practicing with a mirror is extremely helpful for string players. My ideal practice space has a large window that takes up much of the wall. When I hang a mirror the normal way, the hook makes the mirror tilt towards the floor. I decided that the best remedy for this problem would be to prop the bottom of the mirror up so that it tilts ever so slightly towards the ceiling, which allows me to see the position of my bow in the space between the fingerboard and the bridge.
I made a set of two tilting "machines" from things I found in my desk drawer: