As suggested, using air will do the job. Be careful though, high air pressure can force the piston out at quite a rate of knots, keep fingers well away. The best way to use this method is with a G cramp and a piece of wood. That way you can apply air pressure through the brake fluid inlet and release the G cramp slowly.
Make sure the caliper is clean and free from as much fluid as possible. Do this in an area where a bit of fluid splash won't hurt anything. Definitely use eye protection.
You can use the hydraulic pressure from your braking system in much the same way, it's easier if you have someone in the car to apply pressure slowly.
It is very tempting to use flat blades, such as screwdrivers etc to prise the pistons out but this can very often lead to damage of the piston lip.
Remember that the piston needs to come out and go back in, perfectly square [which sounds good for a round hole,lol] but it can occasionally become off-set, a gentle tap with a soft mallet or light hammer and wood will set it true again by gently applying pressure to the high side.
I have always put the calipers in a vice when doing them off the car so you have both hands free to work with.
I've probably overlooked something here as it's just a quick reply but that's the basics of it.
Cleanliness is of the utmost importance upon rebuild.