Body position does not really matter, as long as you can keep enough of an erection (80% or more ) to get a good jelq. I've always been of the personal opinion that a guy should use the "overhand" OK grip when jelqing though. This puts the palm of the hand on top of the penis with the pinky finger toward the glans. This grip puts most of the pressure during jelqing on the sides of the shaft, where it should be, and also keeps the thumb away from the top of the penis, and away from the nerve that can damage erection strength if you damage this nerve.