Looking through the storage drawers for gear I had to pull out a veritable octopus of camera straps. I don't use the manufacturer's straps because I don't want to advertise the type and model of camera I carry. I've tried leather, neoprene, nylon, and canvas. They are all in the drawer. Up until recently I either used the Domke strap from Jim Domke or the Black Rapid sling system. The Domke's are comfortable but I like to sling the camera off my shoulder and they just don't grip enough for me. The Black Rapid dual sling is terrific but the single sling rubs my neck and over the course of time creates an unpleasant state of "man-boob". Neither comfortable or a good look.
I was watching a video cast a while back and Matt Kloskowski and RC Concepcion both were advocating the Upstrap. RC is barrel chested like I am and so I thought I would take a chance on the recommendation and order one.
The strap worked out so well, I ordered two more. The Upstrap is really non-slip. You cannot even make it slide around if you want it to. It holds your camera firmly on your shoulder even as you move and bend, and is also comfortable if you have the camera around your neck. I have both the traditional camera attachments for the Canons and the Vectran attachments for the Leica. All have proper quick release buckles if you don't need the strap and the Kevlar material is incredibly durable.
Best strap ever, may sound like hyperbole, but for me, it is. I'll be ordering a couple more today as I am extremely happy with the product, one for the Hasselblad and one for the remaining Canon. We spend a lot of money on gear. There's absolutely no reason to worry about it slipping off your shoulder or being uncomfortable.