Body down....camera off, and have everything all in place on top of something dust free. The quickly undo one and blast the other on. The make sure the lens you are putting away has been blown off of dust. Insie my house I have a vacumn nozzle by and do a quick suction before changing. Good idea to always try and keep the outside of your camera body clean too.

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."