I have a shooters sling (straps around upper left bicep then sling goes to the forend) and a telescoping shooting stick that I use for off hand standing shots. I also use sandbags for varmint hunting or bipods. I mount a bipod on all of my rifles before I go afield (a 13-27" bipod can be used both laying prone, or sitting) and I hang my shooters sling from my arm. I carry the telescoping shooting stick as I walk.

Even without a shooting stick, with a little practice, you can be VERY steady shooting off-hand just using a standard sling wrapped around your arm. Leaning on a fence post or tree is handy, but I never want to depend on them.