Getting the lighting right is one of the trickier but more crucial aspects of taking better pictures. Even if you don't have a proper lighting rig, you can make a better job of judging lighting effects by imagining light like paint.
Picture by stephduncan
Portrait photographer John Denton passed on that hint (amongst others) at the recent Focus on Imaging conference in the UK. Many people think of light as coming from a single source -- especially if they're relying on the sun or a single piece of lighting equipment -- but as Denton points out, it's more complicated than that:
Light is like chucking a bucket of paint; it will drip after you throw it.
Work out where the paint is coming from, and you can better judge its impact on your pictures. If you can't move the light source, try moving the subject -- not much of an option with landscapes, but a sensible approach with portraits.