I use the furniture polish saturated towelettes (Pledge brand I think) with a thin ruler. It holds up well. Getting the dust down at the base of the strings, where you can't always manipulate the ruler, I work the towelette back and forth under the strings by nudging it from between the strings with a chopstick to rub at the gunk till I can pull it out the bass strings side.

For fluff or crud between the bars, a trick I picked up cleaning a computer keyboard is to use the sticky edge of a post-it note between and remove crud that way.

Have fun with yer autoharp!

Joanne in Cleveland (who has 4 to play and 2 for wall ornament)