if you are going to do rear brakes you need a 'retract tool' to turn and push the hydraulic piston back in,
http://www.lasertools.co.uk/product/1314 I have had one of these for many years and it makes the job easy. There are also little raised pegs on the rear brake pad backing piece that fit into the same radial slots in the piston face as the pegs on the retract tool use, make a note of where the slots are on the piston when you refit the pads (one of the slots horizontal to align with pegs) otherwise the pegs on pad backing plate can case a gap between piston and back of pad, I have no doubt the pegs and slots will eventually twist and line up but much better to try and make sure they do straight away.
The front pistons just push back, but because of the way the handbrake works to rear pistons have to be rotated as you push them back. As pistons are pushed back brake fluid will be displaced back into the reservoir so make sure it does not overflow and make a mess (also a very good paint stripper).
You can use long nosed pliers to fit into slots and twist piston back in (RH thread) but I found the proper tool a lot better. Strangely Halfords sells the laser tool above for about £35.