This is a double-prong problem. Drew is right: controlling the dust is the first step. I periodically blow out the bellows on the enlarger with compressed air. I am very fortunate that our community darkroom has built-in compressor ports; but if I didn't have them, I would still find a way (e.g., vacuum, manual air blaster). All the other advice from using Photo-flo to using compressed air, anti-static brushes, and cloths covers the second prong of the problem. However, I caution against overusing these procedures. I was working with a negative that had some dust. The more I treated the negative, the worse the problem got until a professional photographer friend of mine told me that overusing compressed air or anti-static cloths/brushes actually increased the static electricity.