I am attempting (first time) to replace brake pads on the rear of my 93 honda accord. I got everything off and it seems pretty straight forward. My problem is I can get the caliper piston retracted. I have tried turning it with a pair of needle nose pliers and I have tried a big c-clamp. The c-clamp seems like the best route but I can't get it on the caliper "square" to apply direct perpendicular pressure. What do I need to do?

To provide clearance for the new pads, you should be able back the piston into the caliper bore by rotating it clockwise with a pair of needle-nose pliers. There's also a special tool for this purpose. If the piston doesn't budge, it's likely frozen. It's not expensive to install a rebuild, if necessary. Be sure to bleed that line, afterwards, if you install a new caliper.

Be sure to remove the master cylinder reservoir cap, or open the bleeder screw, before trying to back the piston into the caliper! This is likely your problem. The piston should go in easily with pliers.