you can have a go at pumping out the pistons one at a time, with the disc and pads removed, clean up around the piston as much as you can and lube the seal with the relevant hydraulic fluid as you pump out and return the piston a few times. wipe away excess fluid before reassembly. removing pistons entirely would be good if you can reblead...
+1
take the caliper off the frame / fork
remove the pad
bit of oil / brake fluid (depends on your brakes) on a cotton bud
run this round the lip of the piston
try gently to push the piston back in using a closed spanner say 6mm
if you don't get any joy then you're looking at either needing to change the piston / seal or the whole unit depending on the brake.
the hardest part here is getting the piston out and then re-bleeding, it does give you the opportunity to fit pimp braided cables though
