Style

I've read that the best means of developing ones writing ability is continual use combined with the reading of as many different other sources of work as one can.
If you continue to read more and more books by one person, you're style will begin to reflect the style of those you read, while if you read from different sources a more robust writing ability will emerge. I feel its important to practice writing in styles you don't naturally feel comfortable writing in.
To use poetry as an example, if your favorite rhyme scheme is abab, now and then make sure to write aabb, or free form. Experience the other way's of writing, *flex the muscle* if you will. By doing this you're ability to adapt in certain situations to meet new requirements grows stronger.
If you constantly write in a dark tone, you will fall into the habit, and soon the dark tone loses its impact. If you describe every little thing with a heavy and dark atmosphere, the atmosphere ends up less potent. training yourself to be able to flex into a lighter mood, or a comedic mood is extremely important. Once you are able to do that, you may soften the reader up, before dropping into the heaviness of your message, or vise versa, using dark to emphasize light.
Its the same principle as stepping from a bright room into a dark one, or a dark one into a bright one. After having been in one, the severity of the other increases.