When replacing disc brakes, before removing the caliper, use a screwdriver (or however many pistons your calipers have) to work the pistons back to fully compressed. If your calipers have more than one piston, be sure to compress both at the same time, otherwise you'll just push the compressed one back out.
This worked amazingly last night. I've always fought with caliper pistons in trying to get them compressed. Never again will it be a problem!