Big flathead screwdriver or a small pry bar. Pry on the piston, not the rubber.
I like to open the bleeder when I push back the pistons because a) it lets the old cooked fluid out and most of the cooking happens there for obvious reasons, and b) it doesn't push any dirt/debris/precipitates back up through the brake lines into the ABS which can do Bad_Things(TM) to the ABS module.
Obviously this requires a bit of technique to avoid getting air in the system as well as some extra fresh fluid ... but I find a long piece of clear tygon tube with an "air loop" above the bleeder will allow you to see the color of the fluid, ensure no air gets in, and allows you to contain the mess by not getting brake fluid all over the caliper, rotor and pads.
YMMV.
