Dave recently weighed in on Murray Custom leather rifle slings. While I like a sling on waterfowl and turkey guns, my favorite sling is the utilitarian Quake Claw that several of you mentioned in your comments. The Claw is ugly in black and absolutely hideous in camo. However, it does not slip, and the rubber has a little give to it when the gun rides on your shoulder. Most important, being made of rubber it does not soak up water and burrs don’t stick to it. It is the only sling I will consider putting on a waterfowl gun.
A sling lets me keep a gun on my shoulder all the time, because nothing is worse than having the only ducks of the day try to land in the decoys when you’re out picking them up and your gun is on shore. And yes, ducks do show up when you’re out in the decoys because — I believe anyway — the ripples attract them. To guard against those ducks I sling the gun with the chamber empty and two in the magazine. I usually take the sling off in a blind, as sometimes they get tangled up. I also learned this past week that standing sunflower heads in a dove field have an uncanny ability to snag shotgun slings.
For trudging after gobblers I like neoprene slings which have more give than the claw and really lightens the load of a gun bouncing on your shoulder. When I am sitting and waiting for turkeys I leave the sling attached and put the padded strap on my knee. Then I rest the receiver on the strap. That is much more comfortable than letting the metal edges of the receiver dig into your unpadded kneecap.
For whatever reason European hunters like slings on their bird guns while we do not (Currently there is a cool old German J.P. Sauer double at my local store with sling swivels that I wish I could afford). Although I have never hunted in the uplands with a sling, I could see how once you did, you might really like it. Your hands would be free for taking pictures or fooling with dogs and collars and bells and leashes.