I use a Black Rapid RS-7 Sling Strap:
BlackRapid RS-7 Curve Camera Strap (Black) RS7-1BB B&H Photo but only because I was after one and this is what they had in the store. I've not used any other sling straps to compare it with and found it comfortable during all day use with the Pentax 70-200 on it. The sling gave me access to that lens whilst I also had the 150-450 mounted on a monopod.
So I was after the shoulder strap to use with telephoto lenses at sporting events and of the couple of types the store had the Black rapid seemed to be the best option. Some elements of this strap I like are it's well constructed, the design of the screw gate carabiner with secondary lock for securing the lock nut to, as well as the locking clips that are on the strap. These clips can be locked to keep the camera (or lens in my case) at the bottom of the strap and more stable. Fur use you unlock the front clip to allow the camera to slide up the front strap. The clips worked well from my experience and seem to be a feature on a number of different brands of sling.
Before I'd bought this model I was more interested in one that had a second supporting strap based on the assumption that it would allow the shoulder piece to remain in place on the shoulder and maybe even take some of the load. This is what I'm referring to here:
BlackRapid | Curve Breathe Alas again I can't comment on how effective this would be as I don't have one on mine.
Like Larry indicated above I also use a handstrap, mine is this Joby model:
Camera Hand Strap | Ultra Fit Hand Strap With Ultra Plate which allows me to carry the camera in the hand but still attach the sling strap as a secondary attachment system if I want.This approach is of course not relevant with telephoto lenses as I attach the sling to the lens tripod foot and avoid supporting the weight via the hand strap on the camera.
Judgning from some comments above I might actually look into a seconday system before my next outing.
Tas