I know this belongs in the Bike Mechanics forum but I figured some commuters would appreciate this. I've done it with great success.
I cut the sidewalls (and bead, of course) off old tires (low or worn tread preferably) and insert the remaining tread portion into the new tire as a liner. Obviously the tire you use as a liner must be the same diameter as the new one.
Makes for heavy tires, but if you are having chronic flats I haven't found a cheaper or more effective fix.