If you do your own bike servicing I can wholehartedly recommend buying a Laser 5180 Motorbike Brake Piston Rewind Tool.

When doing brakes it can save you hours of pain & frustration trying to get pistons back where they should be to install new pads.

The best £43 I have ever spent! (Amazon)

P.S. I believe that the manufacturer of this tool is linked to Snap-on (So you're getting a precision quality long lasting bit of kit)