Ammonia. Dilute it and spray it on. Let it sit for a half hour or so then scrub with a brush. You can even use Windex. This will work better than bleach or anything else I've tried, with much less scrubbing. The ammonia lifts the oils from the grout.
By
08/13/2010
Comet cleanser, a tough brislte "denture brush, and a whole lot of elbow grease! Adding a descalent such as clr or like helps, but you cannot get around the elbow grease.
By
08/11/2010
I have been successful cleaning grout with a toothbrush and bleach.