I use a Crumpler Industry Disgrace Camera Strap. The neck strap is wide enough to evenly spread out the weight of a 5Dmk III and a 70-200 f/2.8 lens. I used it to shoot a wedding and it was comfortable all day. Although, I didn't have it on my neck much, more or over the shoulder.
B&H sell it:
I also have a "convenient disgrace", which is the same strap with buckles to remove it from the body, leaving just strap leads (e.g. when used on a tripod or for packing into a bag). I wish the straps were shorter, though, because they still get in the way.