Hi Rick,
A while back a friend was complaining of having the same issue and, jokingly, I suggested he use the double clip hat tether he was waring to secure the sholder belt to the opposite shoulder of his shirt. He responded that "this sliding belt is such a PIA its worth a try." When meeting a few months later, he was still using the hat tether to stop the belt from slipping off his shoulder and suggested we promote them on eBay as Belt Tethers.
Hope this helps,
Ray(64BJ8P1)