If dog jumps at my back, I sometimes stick one of my feet backwards, ie bend knee at right angles, makes the jump a little less comfortable. But essentially just keep turning your back and wait until he stops, jumping takes a fair bit of effort so should tire out, or just lose interest, and the second he stops, turn and praise him. If he jumps again, he gets your butt and no attention until he stops. It's the repeating over and over that helps him learn that his actions have undesirable consequences. He wants your attention, so withhold until he behaves how you want.