A setup I heard about through galopin(?) and which he got from from ldeit(?) comes to mind:
Put a video camera DTL and connect it to a TV in front of you on the floor. Tilt the TV up so that you can see it from your setup - looking straight ahead. Draw a plane line with a whyteboard marker on the screen. Now swing slowly as you look-look-look at your turning shoulder in relation to the line.
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When James Durham recorded 94 at the Old Course at St Andrews in 1767, he set a course record that lasted 86 years. Golf: A curious sport whose object is to put a very small ball in a very small hole with implements ill desiged for the purpose - Sir Winston Churchill