Re: fatigue using neck strap.
Frankly, after years of walking around with heavy film SLR's hanging from my neck, I hardly notice the weight of my M6. Carrying on a shoulder offers relief but raises the spectre of the strap slipping off the shoulder and the subsequent too-horrible-to-think-of disaster.
As a safeguard, I frequently hang the camera from my shoulder and then don a jacket of some sort. Takes a bit of practice to swing the camera up to eye level but it's really quite do-able. A bonus advantage of this is that the camera remains hidden under the jacket when not in use. You might want to experiment.
I have tried Gordy's wrist strap, and it actually remains on my Bessa R (fitted with the 50/1.8 Canon). But for long term carry it doesn't work too well - sort of like being handcuffed to the camera - and fairly impossible to reach into one's pocket when trying to pay for a cup of coffee.
Drewbarb's idea of using the rigid-frame backpack has merit, but only if you're backpacking.
I have a photograph on the wall next to this computer of Henri Cartier-Bresson, published in Northern Journey Photography shortly after his death, and he's holding the camera just in front of his face and there's this curious lacing of the neck strap on his right forearm (I've tried to duplicate this lacing several times and each attempt has failed miserably) that suggests it might allow the camera to hang from the forearm rather than having to be constantly clutched in the hand. Also, a closer look suggests either a filter or a lens cap in his left hand that seems to be attached to his little finger by a cord? Haven't sorted that out yet.
I guess there are more ways of carrying a Leica than I would have thought.
