What is the best way to clean mildew off of marble shower walls without dulling the shine? I have used soap-scum removers in the past, but they only dull the marble.

There are several products on the market that will remove mildew from tile, but they will also etch any polished marble surface. I suggest the following technique:

Dilute one cup of household bleach in one-half gallon of cold water; add several drops of liquid dish soap. Wipe this solution on the mildew and let it stand for fifteen minutes, then rinse with plenty of cold water. If this does not remove the mildew, increase the standing time and repeat.