How can I get my ceramic tile grout to come clean?

The grout between my ceramic tile is severely stained. The only thing that gets it clean is scrubbing with a grout cleaner from Home Depot on my hands and knees with a Sonic Scrubber. It takes all day, and only stays lean for a few days. I will not do that again. Does anyone know of a cleaner that will get the grout really clean just by mopping with it? When I mop with regular mopping products, there is no change in the dirty look of the grout. Thanks.