I suppose there's a professional way but what I've always done is:
Get it before it dries absolutely rock hard IE 12-24 hours
Use one of those green semi-scrubby cloth/scourer things, (like a thick hairy mat) and something like liquid Ajax or floor cleaner (avoid dyed ones)
Just add elbow grease
If the tiles are ceramic glazed, ie shiny you can buy coiled up steel scrubbers in wilkinsons and most pound shops with water and slightly lees elbow grease you will get all the grout off.