I find that mixing some baking soda and Vinegar together in a spray bottle and then spraying the baking soda and Vinegar mixture onto the soap scum and letting it sit for a few minutes helps to clean the soap scum right off the glass shower doors.
You might also want to get yourself a magic eraser as I have one and it helps along with the baking soda and vinegar to remove the really stubborn soap scum from my glass shower doors.