Many of us use the crop tool far too sparingly. With modern cameras having such high megapixel counts, you can afford to experiment with cropping your image without diminishing your photo quality. If you find your image is a little boring, see if a tighter crop on your subject helps your composition at all.
Look at the edges of your frame and ask yourself if the features that are there add to the image at all and if not, get cropping. Fewer distractions will actually lead to a more striking and successful composition.
With certain styles of photography, such as candid street shots, you can have more power if your images aren't perfectly straight. In other cases, straightness is essential. To check if your image is level, turn your attention to the horizon and use the ruler tool for help.