Don:
The best luck I've had with drybrushing is a brush with short, firm bristles (nothing that is wispy), with the bristles cut at an angle (in other words when you look at the bristles when the brush lays flat, one side is longer than the other side). The width of the bristles on the entire brush is probably no more than a quarter inch.
If this helps, here is the id on the brush handle, 1/4 Americanpainter 4400 Angular Talon.
I've used this brush over and over with good results--ie good control, and with the bristles being relatively short and narrow you don't pick up a lot of paint to clean off before drybrushing. Its cheap, only a couple bucks.
Hope this helps.
Steve aka WeWillHold