It really depends on the style of floating shelf you're using.
There's one where you mount a long "C" channel (well actually, it looks more like this [ ) to the wall and the shelf pushes in to the channel. This style comes in metal, wood and glass. It looks really nice because the mounting is invisible and the shelves are generally thin (about 1/4"). It's good for holding smaller, lighter props.
There's another style of floating shelves from Ikea. These have a long flat bar with evenly spaced poles sticking out of them. These poles slide in to the back of the shelf. These only come in metal and are generally 2" - 3" thick, depending on the shelf length. These are big and ugly but can hold a lot of weight.
Then there's a differently option. It's not a true invisible mount, but it does camoflauge nicely and can hold a lot of weight for it's size. These are the type I use - John Sterling Corporation, designer shelf bracket
It's basically an L bracket with a plastic cover. Depending on how much weight you need to hold, you can add multiple brackets. The cover can be painted to match any wall color, hence the camoflauge.
Hope it gives you some ideas.
Thanks to WC for hosting.
-Fred
