When you open and close Petri dishes for bacteriology you learn to minimise any chance cross-contamination by spores and airborne bacteria by having the lid plate always pointing its inside surface down and turning over the bottom plate so that the agar is pointing down. That way, particles do not settle on the inside. I do the same when changing lens: take off the rear lens cap of the new lens and place the cap with the open end down and the lens with the open rear down; twist off the body and place it facing down; immediately put the cap on the removed lens; reassemble the camera and the other lens. I never have problems with dust on the sensor or in the rear end of the lens.(And I use the 100-400mm L and 24-105 L, which supposedly suck in dust.)
Very good advice as usual, Alan...but doesn't it presume that you are changing lenses on a very clean surface to begin with (by laying the lenses and open body facing down)? For instance, if you're changing them on a dusty car seat, or a pollen covered park bench or something...it wouldn't be as good as changing in a cleaner setting.
I recall seeing a lady changing lenses at one of my nephew's soccer games a few years ago. She obviously didn't care much about sensor cleanliness...the camera body was facing the sky, and it had started to rain. The mirror of that camera, wherever it is now, is probably covered with water spots and no telling what else.
I too mentioned that the 100-400 could suck in dust, in another thread, and was promptly told that I didn't know what I was talking about. But I do. I recently bought the 24-105. It has a bit of dust sealing at the zoom juncture, but this obviously wears out over time. Mine is still tight, but the one I rented last year, was pretty loose. Once it stops being tight, there's a bigger crack for the dust to get through.
As for mirrorless cameras leaving the shutter open and the sensor exposed all the time...I did not know dattt...I enjoy learning new things every time I come on this website!!