Sometimes I use a BBQ skewer inside a dusting mitt for marginal success. Or take it outside and give it a good brushing with a soft paintbrush reserved for the task. Too bad this typically results in bits of brush bristle stuck in the tight places. Some players believe a little dust is advantageous ... minimizes the glare of bright lights on the sound board!
Check This Out ... Here's a nifty accessory that really seems to work. One of my students showed it to me at the Winston Salem Dulcimer Festival. The bristles are black and white, hence its funny name - Skunk Tail.
String Fever Music website. Just scroll down the accessory page until you find it.