I did my front pads not too long ago and used channel-lock pliers to compress the piston. I just ratcheted them out to the widest setting, slipped them over the edge of the piston and the back of the caliper and squeezed like hell. It finally moved and slowly compressed back in.
You can't really break it so if it is moving with your c-clamp method (which is probably better than my method), just keep cranking. I hope you are using a bock of wood over the piston's edge rather than using the c-clamp on the inside backing of the piston as I don't know how strong that back face is. You might just be able to use the clamp on the edge of the piston (that's what I did with the channel-locks) because that should distribute the force better, even if it is off-center. Good luck!