If that doesn't work, then let the corners of your mouth turn down. Ever noticed how some people get old and seem to have a permanently sad expression while others seem to look happy even when they're at rest? It's those mouth corners.
Well, here's another one, a bit old fashioned but it works: wring your hands. It physically expresses powerlessness. We do so much with our hands. Flinging them about says, "I can't do anything useful, I'm helpless."