I've used the special tools as well as just a C-clamp and I'd have to say that the C-clamp is just as easy.
Actually, I've done it by hand without even a C-clamp before. I put my 19mm wrench across the piston, put my palms against the wrench, grabbed the back of the caliper with my fingers and squeezed. It worked to push the rear caliper piston back in, although somewhat slowly. It's defnitely not the right way to do it because you could potentially push it back in unevenly and cause binding. All I can say is that it worked for me.
