I use a mirror for practicing. It's made of three very thin and highly polished metal plates, hinged with duct tape as others have suggested. These are "unbreakable" mirrors that they sell for public restrooms and other vandal prone areas. Works great for this, and you have never to worry about fragility. When folded, it's only about a 1/4" thick, mostly due to the duct tape.
For rehearsal (not the same as practice, in my opinion) I also prefer video. Lately I've been experiments with using an Apple iSight camera instead of my camcorder. (You can record using iMovie or QuickTime Pro 7.) It's handy too if you want to check angles on something you're working on, the self-view video window in iChat even reverses the image for you, if the left-right view of a camcorder monitor bothers you.