I agree, it's a pain to change lenses when you have nowhere to put
something down, and especially if you are in a hurry. You start
getting to the point of just carrying a second body with another lens
attached...
For me, two things are essential to doing this while standing:
1. Camera on strap, hanging from neck.
2. A camera bag which is hanging from shoulder, and opens on the top
(rules out backpacks for the most part).
Figuring that I want to minimize the time that a lens is off the
camera (dust control), I prep the lens that is going onto the camera
first. It helps that I have big enough hands that I can remove the
back lens cap with one hand.
Basically, I work out of the camera bag, removing back cap from new
lens. The lens is lying in the bag at this point. I then remove the
old lens from the camera and drop it in the bag. (Generally using
two hands for this). I quickly pick up the new lens and mount it,
and at that point take off the front lens cap. I then, if in a
hurry, just flip the bag cover closed and ignore the old lens in
there. If not in a hurry, I put the caps on the old lens.
Often I will have 2-4 lenses in the bag, kind of jumbled together on
top, with no lens caps on either end (but usually with hoods
attached, which helps protect the glass). I grab whichever one I
want and swap. When things settle down, I clean up the lenses in the
bag and put them away in their various compartments.
I find it a lot easier to carry a D200 with one lens on one shoulder
while having my D300 on a neck strap with another lens. But that
certainly has other issues, not the least of which are straps getting
all tangled up around the neck, and the sheet weight.
Helps to have straps with quick disconnect buckles to put stuff down
and sort it out.
Anyway, if you have a shoulder bag, I think you will quickly end up
using it as your quick (& dirty?) table. At least your lenses will
be relatively safe from rolling off a table, getting dropped, and
protected from rain while you swap.
