Here are a few general notes to keep in mind:
Strong silhouettes make the image readable at a glance. Use them to drive the attention to your focal points. If two elements are too close in value, paint something between them to better define their position in space. Smoke, light or fog is usually a good solution (Fig.08).
Always check your value structure and use it to create the illusion of depth in your environment. To help you with this, put a black and white adjustment layer on top of your painting and turn it on every now and then to check your values. If something doesn't look right, it's probably wrong. Fix it before moving on to anything else. The picture below shows you a value breakdown of the painting (Fig.09).
Try to think about contrast every step of the way. Big vs small, dark vs light, horizontal vs vertical, hard edge vs soft edge, etc. If you have too much of one, add some of the other. This rule applies to brush strokes as well; play with different sizes and directions.
After a couple of days of work, here is the final result (Fig.10).
That's all guys! I hope you found this tutorial informative. If you want to know anything in particular, please don't hesitate to shoot me an email. In conclusion, I'd like to thank James Paick for his advice and critiques. Without his support, this picture wouldn't exist.