I often walk around with my camera on the lookout for shots, but photography isn't the main thing I'm doing. I normally end up with the camera hanging off a shoulder (on the strap it came with), which isn't the most secure way of having the camera. The camera and lens would be too heavy to be comfortable just around my neck, and might bounce around whilst walking.
So what I'd like is to find a strap I can have across my body so the camera can sit to one side, with an arm over it to offer some protection. I'd also like it to work (ie be easy to raise to the eye) while wearing a rucksack over the top. Any recommendations?
I have a Canon 40D, in case straps aren't a generic thing.