The biggest tip is that you screw the piston clockwise into the housing rather than just push it in with a g clamp. Unfortunately sometimes the piston still doesn't go in so you need to get a bit agricultural and hit the piston with a hammer and then screw it in. You'll need to do this several times before it's wound in far enough to get the new pads in!
The tool you get for changing an angle grinder disk can be used to screw the piston in and I've seen Halfrauds and other Motoring shops are now stocking the proper tool for about £15 (I think) which may remove the need for using a hammer!
