A contractor friend taught me a neat trick for cleaning white sheet rocked walls. You need a pine/green cleaner (non toxic one's do exist) and a rough sponge (often green). Spray a bit of pine cleaner on the sponge and scrub. Wala!
Small photos = before and after
NOTE: the brand in this photo is not one of the non toxic brands. Whole Foods has one and there are others, look around.