There are many ways to improve your compositional skills. Knowing ahead of time what you are trying to achieve and what you want your image to look like though can help tremendously.
Pre-visualize how you want the image to look before setting up your equipment. Think about how you can convey your feelings about the scene on film (or digital of course). Decide which compositional tools would be helpful and then put them to use to make it happen. The more experience you gain from working with the techniques presented here and elsewhere the easier this will become.
Ansel Adams was of the pioneers of this technique as part of his Zone system. While working as a photo-muralist for the Department of the Interior in Washington D.C. during World War II, Adams started formalizing much of what he ad learned from his years of photographing in Yosemite and the Southwest. One of the cornerstones of the Zone system is to pre-visualize the final print and determine what would be needed to best realize that result. This then would then become the game plan for what he did in the field as well as in the darkroom.
In order to be able to do this though, you need to know your equipment well. You need to know that if you use this lens in that way and shoot from this position that you will get that result. You need to learn how your camera and lenses see and begin to predict how viewers will react to possible ways of rendering your subject so you can quickly come away with an effective way of showing it.
Pre-visualization is almost a requirement to get truly good images consistently. Without it, you are basically shooting without a blueprint. Before buildings are built, an architect always prepares a blueprint. So should you.