The other day I was astonished when I watched a person change lenses with no concern for keeping dust out of the camera or lenses. I have always had a ritual, but now I am wondering how others do it.
1. Get out of direct sunlight; put lens cap on old lens.
2. Unscrew (but don't remove) the back cap of the new lens, stand the lens (rear end) up with the rear cap laying on top.
3. With the camera pointed to the ground (to keep dust from falling into the camera) remove the lens and place it next to the new lens
4. Place the rear cap from new lens on top of the old lens and put the new one on the camera that is still pointed to the ground.
5. Screw on the rear cap of the old lens and put it back in the bag.
6. Remove the lens cap and shoot.
I do steps 3 and 4 as quickly as possible to minimize the amount of dust getting, first into the camera, and second onto the rear of the lens.
The person that I watched was carrying on a conversation while she changed lenses. She had all of the lens caps off and laid the camera on a table facing straight up while she dug around in her bag. Watching her drove me nuts!