This is for use with the PET Bottle Rig
Instead of using a balled up piece of tape, a pebble, or some other thing to hold down my camera's shutter, I've gone to using a knot. This makes it easy to hold in place, and makes setting it easier. I prefer a rubber band, but tape also holds it down. tape may be more useful for bulging, non rectangular cameras.
My Canon SD1000 needs the rubber band tripled up to hold it in place: