This is how I do it the old way:
Take out the glow plug, Get a piece of string, can be made of cotton or fiber or just something that would not shed (not too thick and not too thin either), tie a small knot at one end, make sure the knot is small enough to go thru the glow plug hole, then insert a small length of the string, then turn the cranck shaft in the way to loosen it with the knock nut and the piston will be stopped by the small piece of string before it reaches to the top. Now you can un-screw the knock nut. After you are done just turn the piston back to lower position and tack the string out. Sorry for my poor english and Hope this helps.
Hint: There is a tool called Piston Locking Tool I think and it will make this step much easier, and good for working with flywheel. Example: http://www2.towerhobbies.com/cgi-bin...?&I=LXKTM7&P=7