About avoiding lens changing:
When I first got the D70 I was trying to do this. I have given up. I still try to be more careful than I used to be with 35mm SLRs about pointing the camera down, not changing lenses in very dusty places, and not doing unnecessary lens changes (e.g., I leave whatever lens is on the camera on it until I need another one, rather than changing to some standard lens just in case), but there are lots of well-documented techniques for cleaning sensors; I have used both blowers and swabs now and the blower works sometimes, when it doesn't the swabs do. There's no point in having an interchangeable-lens SLR if you aren't going to change lenses, or only change them under really desperate need. I have only once had a big chunk of dust land on the sensor during a trip that was a bit of a pain to clone out later, and by changing lenses pretty freely I have gotten lots of nice shots I might not have otherwise, which is the point of the whole business.