How to Clean Marble Question

How do you clean the grout in a Marble (http://www.lightemprador.com) tiled shower?  We have a newer home with a Marble (http://www.lightemprador.com) (honed calcutta gold) shower where the grout seems to be getting "dirty" even though we are using a squeegee to dry the walls and (http://maplered.com) floors after every use.  What type of product should be used?  Something different for daily/weekly usage vs. occasional heavy duty cleaning?  Thanks!