Hold the phone in landscape orientation (with the volume buttons facing down the way), now grip the bottom left of the phone, the opposite side from the camera. Doesn't even need to be a death grip, just clasp it a little. Unless you're a few metres from the wifi base station you'll watch the Wifi signal drop and eventually disappear completely. Should only take about 10 seconds. Apparently the iPhone is always going to have horrendous reception problems of one form or another.