I have the same problem. I haven't used an official sling, but i've done the belt/waistband/fabric sling/carrying my L arm with my R arm tricks. Standing for a few minutes hasn't been a problem, but standing for an extended period.....at church or going for a walk is.

My worst is when I use both arms....even for light activities. I've been to a few parties with beverage in right hand and cigarette in my left. It's my fun times......but I'm in bed for days after it. Obviously, I get no sympathy from my docs for that