Changing brake pads - need help NSFW

[CrazyEyez]

I've got the new pads in, but I can't get the piston pushed back to fit the caliper over the new pads. What's the trick? I don't have a c-clamp. I tried putting a piece of wood over it and rapping it with a hammer - the thing won't move.

Please don't tell me I have to drain brake fluid.
Edit: I say that about the fluid because I assume it's a big mess/hassle. If that's not the case, please enlighten me.