I use a
wood clamp. Before you remove the caliper from the bracket, hook one end of the clamp behind the caliper and use a socket as a spacer on the bracket. Then you can just ratchet the clamp shut; it pushes the caliper pistons against the rotor and forces them back into the caliper. Since the caliper is sliding on its pins and it's pushing against the rotor, the pistons get pushed straight in and there's no chance of them getting cocked in their bores.