I use the Brownells Latigo sling for my hunting rifles. They can be quickly and easily adjusted to any length you desire.

This allows me to pull it tight when I'm sitting or walking with it unslung. This keeps the swivels from making any noise. When I want to sling my rifle I just pull down on the outer part of the sling to the length I need and up it goes.

My packs have full padded shoulder straps with mount points, not just a thin nylon strap like many packs I see. I have cases attached to each of the straps at my breast - one for my radio and one for my GPS. To keep my rifle on my shoulder without slipping, I put the sling towards the center of my chest, whichever side I sling on has a case to catch the sling and hold it, keeping it from slipping off.

I still use the first Latigo sling I installed back in '93 (I've used it almost every year since then). It's held up amazingly and doesn't slip loose when I don't want it to.