Peter Krebs reccomends soaking in Coka Cola for a couple of days and then toothbrushing with clean water. Says it works like a charm. Haven't tried the Method yet but there's one or two pots I need to clean this winter.
The best method is prevention! Rubbing the pots in use down with oil(many varieties will work, even veggie!)
Every week or two will prevent limescale and improve patina! Win win. I have pretty hard water and used to have a real problem with buildup, but no longer! Plus you spend a little More time with trees you may not be focused on at the moment, catch problems earlier and tweak more often.http://japanesepots.wordpress.com/