Well, standing under a tree in a thunderstorm is probably the action you mention that has the biggest problem. Trees are often the tallest objects around and contain many pointy tips. The electric field is strongest near the pointed tip of an electrical conductor, and so trees often make for great targets for lightning strikes. Trees don't always conduct electricity as well as you do, since people are made mostly out of saltwater. The current from a lightning strike may leave the tree and jump over to you and follow your body on its way to the ground.
Standing out in a field far from trees may also pose risks too. In this case, you are the tallest object around and may be a target for lightning strikes. If you are carrying metal golf clubs at the time, it could raise your risk (this is a common reason people are standing out in flat fields during thunderstorms, is to carry golf clubs. Sometimes such people get struck by lightning). The safest place to be during a thunderstorm is a building or a car. Electricity can flow through the metal sides of a car and not through you.
I'm not so sure about your other questions. Standing with legs together or apart may not make much difference on anything. The only thing I can think of that it might have an effect on is if there is a nearby lightning strike, and there is a large amount of current flowing along the surface of the ground nearby. The ground has a resistivity, and so the voltage changes rapidly from one place to another during the time the current flows. If your legs are apart, the current may decide to flow through you rather than the ground, but this depends on how good an insulator the wet ground is and how good an insulator your boots are. I'd imagine that this is much less likely to cause injury than a direct lightning strike anyhow.
Lying down actually sounds like not such a bad idea, except that during thunderstorms it is probably wet down on the ground. It's best to seek a house or car to stay in during the storm.
(published on 10/22/2007)