An upslope lie has the effect of making your left side longer, which can tip you back and lead to poor contact. To compensate, drop your left foot away from the target line and bend your left knee more.
Opening your stance like this pre-sets your turn through the ball. You'll need the help, because the upslope makes it tough to rotate toward the target.
Why do you want a body turn at all? Because you need power to pitch uphill. The slope adds loft, so you tend to leave the ball short. But I still like a sand wedge here, especially in heavy grass.
Another benefit of setting up open is that it promotes an out-to-in swing, which is steeper and gets the clubhead through the grass and under the ball.
Jim McLean, a Golf Digest Teaching Professional, is based at Doral Golf Resort & Spa, in Miami.