+1 on a using a large C-clamp to push the caliper piston back in. Use a thin piece of wood or one of the old brake pads to cover the piston so you have a nice flat surface to push on with the clamp. Cover the back of the caliper with a rag or something soft to avoid damaging the surface of the caliper (especially if they are painted like mine are).
Open the brake fluid resevoir (tank in the front trunk) to lessen the pressure as the fluid is pushed back into the resevoir as you push the piston back into the caliper. Be sure that the resevoir doesn't overflow as you're pushing the piston back in or you'll have a nice brake fluid mess in your front trunk.
It might take some pressure on the piston to get it to move back into the caliper but not a huge amount. Try using a
slow and steady pressure so the fluid can flow smoothly back into the resevoir - if you try to give it one big push it ain't gonna move at all.
If you can't get the piston to move at all, then you may have a frozen piston and the caliper will need to be removed and rebuilt. This is pretty rare.
This is a pic of the general idea of using a C-clamp...
