I use the netting of a kitchen sponge at the momment, and thats rough enough to get all the gunk off. But I do that every 2 weeks when I change strings. I've been wanting to get some of those plastic wool things also used in the kitchen, it would be easier to get the stuff off with. I've heard about people ussing steel wool, but I'd rather not use that, it seems like it might messup the wood. Its also good to get some Fast Fret and apply that about twice a year after you clean the fretboard.