A good retention holster and a backup gun on the weakside.
I don't see this as completely necessary. I always carry a knife on my weak side. I have had retention training, and knife training as well.
I have had someone go for my gun once, and it was out on my farm. We had bought two horses from a guy and he had come over to check up on them. I admit, when I am out there I let my guard down at times, and I did on the day that he was there. He tried to grab my gun (later he said "just to look at it") and I responded and kept my gun. He did not know how to operate the SERPA so it was a help as well.
He was escorted off of our property, and has not been invited back.