I'm using BlackRapid RS-5. It's very comfortable, can hold quite heavy lenses (I'm sure even 500mm f/4 could work if you want to hold it while moving around a bit between shots).
I've been using it a lot at motocross events and football matches holding a Pentax K-x and a Pentax 60-250mm f/4, and I've had no fatigue from having it hanging over my shoulder, the last motocross even lasted from 09:00 to 18:00, and it was just fine, of course I didn't stand the whole time (though my feet felt like it <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/tongue.gif' class='bbc_emoticon' alt='
' />) but I was standing for at least 7 hours, and when not shooting, the camera and lens hung by my side. With the 60-250mm attached I mount the RS-5 to the lens not the camera, but using another smaller lens, I attach the RS-5 to the camera.
I have a Manfrotto 394. It's a quick release plate with similar to the one on my tripod head and a base plate to attach it to. This gives me two quick release plates, one that I screw to the stripod mount on my 60-250mm, and the other to the bottom of the camera. Then I mount the base plate to the RS-5.
With a setup like this I can switch quick and easy between mounting the camera or lens tripod mount to the RS-5 or my tripod head.
But when I'm gonna be using pretty much only the 60-250mm, i connect the RS-5 directly as it doesn't take so much space under the lens so I can hold it with out having to grab around the Manfrotto 394 to adjust the zoom ring.
BlackRapid have several models and I'm sure you can find the one for you.
http://www.blackrapid.com/