What Chaos said.
I just changed out my tires and wedged my pads apart with a wooden wedge. The movement in pads is generally very small, so if the pistons are backed out a good bit, it takes a bunch of pumps to get contact again. Just make sure you don't diminish your fluid level to the point where it is sucking in air, which shouldn't be a problem, because you had enough fluid in there befor the work........it doesn't just evaporate.