Others have mentioned 'dead hangs'. I did them a lot when I was rock
climbing and got fairly good at them, minutes. I helps to have a low
body weight, your shoulder socket does not grow like a muscle does.
And never with additional weight. A huge mistake in my opinion, asking
for trouble unless done properly.
If I may so bold, I am going to jump in here on this subject and share
something very important . . . too me, at least based on my experience.
Maybe this is self evident (it was not to me).
Do not hang with all your weight on your shoulder socket. Big problem.
When you have have some tension in your arms to take the load off you
shoulder socket. This will prevent injury's. Do not 'hang' there with all
that weight pulling on your shoulder. Once you have learned that have have
increased your time (my goal) you can always add weight but stick to the
correct protocol.
This is all my opinion of course, based upon my direct experience and the
experience of others including very accomplished rock climbers.
And DC is right on when it come to the 'broom stick' stretch. It is magic.
I have zero shoulder issues despite all the off the wall dumb shit I am
capable of.