I did mine last weekend when I discovered the dust seal hadn't been fitted right by the last bodger and the piston was rusty causing horrendous drag after hard braking.

Anyhow unless you've got 20 tiny fingers on each hand it's hard to get that seal fitted to the caliper and stretched around the piston simultaneously, so I made a tool by cutting off the end of a mastic tube, 1/2 inch long and using that to force the piston end of the seal open while you fit the other end to the caliper. 

Then the piston will fit through the mastic tube and into the caliper to trap the seal in place, then you remove the tool.  Ease the piston into the caliper and the seal will pop nicely into its groove.  Simple pimple.

Thought I'd share my knowledge ;)

Andy