My philosophy on composition is that if it looks good to me when I frame it, I shoot it no matter what rule it appears to break. On a more practical note I'd endorse Eric's comment on looking at images made by other photographers and without directly copying them you will always be able to take something from them to help you develop a way of seeing.

Another excerise I encourage my students to do is to spend time looking at their own images to see how many new images you can find within them. This is the only time that I ever encourage anyone to use two L shaped pieces of card.

Finally, when you are photographing, photograph the light. It may be an obvious statement but it's surprising how many photographers don't do it.