Love the magnet idea! Think I'm going to try that myself. I just make some small cuts along the edge of my stencil, doesn't really matter what shape, but make them about an 3/4 of an inch and 5/8 wide... every few inches, or so....then I just use blue masking tape, the kind specifically made for delicate paint, and put it over those holes and put it in place. I make the cuts so the are only a quarter inch back from the edge. Its not 100% perfect, but with a little care, it works. I haven't had a problem yet with any tape residue doing it this way. I don't mash the tape down on my art either... I just want it to stick just enough to hold it there. I'm guessing if I really rubbed it in, it might do some damage.
If that isn't clear, and you don't get any better ideas, I'll post some pictures the next time I use this technique.... but I really like the magnet idea a lot