...by a lithium niobate anti-alias filter. As long as you take care not to use any hard objects when cleaning this filter, there is no danger of damaging it. Several threads on this site describe in detail how to construct a cleaning device for safely swabbing the sensor assembly to clear it of dust. (You need a rubber spatula--and some simple carving skills--PecPads, and some Eclipse solution.)
I made it about 6 months before dust started creeping into my D100. For me, using the baster bulb simply moved dust around or created even more dust specks. A quick swab removed all the specks, large and small. I now clean the CCD about once a month, whenever it is in need of it. It takes about five minutes to set up and about 5 seconds to clean the CCD.
It would be nice to think that putting some sort of dust shutter in front of the CCD would solve the problem, but unfortuantely, it probably would not work. Every time the shutter opens to expose the CCD, the contents of the camera body, including dust, will have access to the CCD surface. One way or the other, it will eventually get dirty. Probably the best strategy is to make the CCD easy to access for routine cleaning. Good lens-changing technique does reduce contamination, and should be standard practice with a digital camera.