To to be all about me, but I refuse to use a strap. My friend's guitar teacher said... no strap...nerdy?? I agree.
I sit to play and when standing, I guess I am so used to is slipping occasionally that I will time my playing with re-adjusting it in my arms. Anyone who plays the accordion will know that you have to time the bellows so that you don't run out of air in the middle of a phrase. After a while, this becomes second nature and you just focus on the playing while it happens automatically.
Check out the other thread about non-slip surface mat...That might be your answer!