I mostly use primes and find myself changing lenses many times during an outing. Unfortunately, on an outdoor hike, there is not always an ideal place to do this, so I find myself squatting down to do the swap in my lap or I may use my sling bag to create an artificial table at my chest.
Regardless, I saw somebody do this before and have adopted the technique of using one hand to hold the camera and lens release button and the other hand to hold BOTH lenses. The key is to quickly remove the end cap and line up the orange dot on the lens so that when the lens mount is pointing away from you, the dot is at 9 o'clock.
It is hard to explain so I made this little video to show a few lenses being swapped on my K20D.
[YT]http://www.youtube.com/watch?v=U0YUJgwwsAA[/YT]
Whenever possible, i aim the camera downward to minimize dust intrusion. Also, I didn't show this in the vid, but I will typically palm the endcap with my right hand before grabbing the camera, then once the change is complete, let the camera hang freely from my neck and twist the cap on the lens going into the bag.
While I rarely have any accidents, I would go very slow using this technique with an expensive lens. Note that in my video I demonstrated using lenses I do not actually ever use, just in case.
However, the next time you are standing in line at Disneyland and must change lenses or on the beach with dust and sand flying everywhere or in a light rain, this quick change method can come in very handy.