Rule of Thirds
One of the best-known and most effective compositional techniques is the famous Rule of Thirds. I've covered this idea in some depth in a previous tutorial but it is especially relevant to landscape photography. Rather than having the subject in the centre of the frame try to position the elements of your shot so that horizontal lines such as, obviously, the horizon, and vertical elements such as trees, divide the frame into thirds, like a noughts-and-crosses (tick-tack-toe for our American readers) grid. Frame the shot so that major elements of your picture fall on these grid lines, and particularly where the grid lines intersect.
Take another look at the simple shot on the first page, of the house in the wheat field with the sea in the background. In this image, the horizon divides the frame along the upper third line, while the house is positioned close to where this line intersects the right third line. There is some shaded detail in the wheat field close to where the bottom third line intersects the left third line, balancing the composition. It's simple but very effective.