Jeri's post is the best use so far.

For all aspiring luthiers, using duct tape is the simplest way to glue up a bookmatched guitar top. Lay the two halves side by side and stretch strips of duct tape across them. Then open them up and apply the glue. Then fold them back again, and place a weight on top, just enough to keep from springimg up. The duct tape will provide just the right amount of clamping pressure.

btw, the stuff you buy in hardware stores nowadays is crap. You can still get "real" duct tape from specialist suppliers, like Veritas and Woodcraft.

Murray