Ralph has iterated one of the mysteries of the universe: That's why you need to keep everything as dust-free as possible, since even if you have only one speck of dust, it will end up in the sky, or some other continuous-toned portion of the image and be a PITA.
Seriously, keeping things clean when loading holders is essential for dust control. Clean the holders well before loading paying particular attention to the light trap. I vacuum mine. Pick a dust-free place to load. You don't want a lot of dust floating around in the air or try to load holders inside a dusty changing bag. In my darkroom, I keep a small air-filter running. Settle the dust by increasing the humidity. I run some hot water a bit. Once you are ready to load, try to minimize the emulsion surface's exposure to possible dust contamination. I start with my film stack emulsion-side down and turn the film over to load. Close the darkslide immediately after loading.
Once you have your holders loaded, get them into individual zip-loc style bags to keep dust off them. Before shooting, make sure you have dusted out the inside of the camera bellows (don't want dust floating around inside the camera either). When you remove a holder from its bag, check again for dust on the holder surfaces (there always seems to be some!). Brush or blow it off before loading. After exposure, dust is not such an issue, but any dust on the negative at the time of exposure will block light to that area (i.e., the sky), causing a "pinhole," which will result in black speck on the print, which is not fun to spot or etch off.
Hope this helps.
Doremus
www.DoremusScudder.com