I am religious about doing my best to keep the D200's sensor dust free. When I'm changing lenses: prepare the replacement lens to reduce changing time; remove camera/lens from tripod; point camera/lens downward; remove lens; attcah lens; remote camera/lens. If windy or windy in a dry dusty area, stand with my back to the wind before staring the process. At the end of the day, I clean all lenses, lens caps, and the body cap.
And the sensor still gets dirty. The D200's sensor has been professionally cleaned once already; the tech showed me how to clean the sensor; and I bought the sensor cleaning kit, which I will be using for the first time.
I can easily fix the spots in Photoshop. But being the OCD poster boy, I'm becoming pretty obsessed about this.
I'm with Alan here - clean only when necessary. I use the same lens changing procedures that you do. The only additional thing I could mention is that I once read in an article that there was a benefit to storing the camera sitting upright or with the sensor facing down - to avoid storing the camera with the sensor facing up. I have no real evidence that this has helped, but I have had my D200 since December 2005 and I have used the damp swab cleaning method only once (about 6 months ago). I found that simply using a Giottos Rocket blower on the sensor occasionally would remove the easily-visible dust spots. (I didn't worry too much about tiny spots off in the corners of the frame.) It wasn't until a few months ago that I finally got a stubborn bit of dust near the center of the frame and so I had to resort to the swab method.
Of course, it is also entirely possible that I shoot a lot less than you do and my sensor is exposed to less dust.
Of the three cameras I have the D200 seems to collect more dust than the others. In a visibly dusty area I might hesitate to change lenses, but short of that I don't worry about it any more than I did using my film SLRs.
When you notice the dust in the pics, clean the sensor. Dust will happen. It is a fact of life.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA
I also use the rocket blower when I have to but not often the dust is in my D200. with zoom lenses you can have a problem you suck the dust into them and that might come into the camera too. Dust is a common thing, so don t worry a lot.