I replaced my brake pads today and while in the process of twisting the rear caliper piston clockwise, I rounded the the center portion of the "X" with my 3/8" extension. A short while later I figured out that you can also use one of the old rear brake pads to get the piston twisted into the housing, just remove the old shim.

Use the alignment key end of the old brake pad...it fits perfectly. To get the piston started you will want to also use a pair of pliers to hold on to the end for enough torque to get the piston to twist, after that you can just use the pad itself.

 

Lots of different ways have been posted, but that's a new one to me. Good idea...