While art is pure creation, pure self-expression, professional digital art needs both skill and an efficient/effective method of workflow to get it done in a timely manner, and to assure the final product turns out professional in quality and effective in appearance. To learn the best method of workflow, it's best to learn the methods with which professional artists work. For that, I do suggest you go to The Gnomon Workshop to pick yourself up one or more training DVD's featuring lectures by Real Professionals working in the industry.
In the meantime, here is some quick information about the methods I've learned from them that have helped my art skills and techniques.
First, you need to come up with creative ideas fast. The best ways of doing this are to either draw, or print up, a bunch of small rectangles on a piece of paper and start scribbling in quick images to get Thumbnails of ideas you can blow up later, or to pull up Photoshop or other painting software and start sketching various quick rough sketches of art ideas. Remember to focus on Form and Composition first, rather than on Detail.
Then, take one of those ideas and draw, or digitally paint, a larger, more detailed version of the image, focusing first on basic shapes (form) and Composition, then on Lighting, and gradually more and more on Detail. Answer as many questions as you can so you know what's expected of you when creating the Final Image later. If you worked on paper, once you've done as much detail as you feel you need, you can scan it into the computer, pull it up in Photoshop, and begin digitally painting over the image until you get a final digital sketch.
Whether you started on Paper or in Photoshop, once you have your final digitally painted Sketch, and you've answered all the questions you can about lighting, color, texture, and design, it's time to start building the Final Image. You can either blend pieces of photographs together to finish the scene, or you can blend rendered images of 3D Models with it, or you can build the whole thing in 3D and composite it later in Photoshop or an Effects Compositing Program. It's up to you.
Then, make sure your Post Production is professional in appearance also. I usually frame my posters in a blur of the colors contained within the image to give it a glass or metal feel that flows with the image. I don't typically use an all-black or all-white frame, but I have seen some artwork framed with all-black that looked very professional. So, it really is up to you. But watch to make sure your Title isn't too large and overbearing, or made in some kind of cartoonish or goofy font, as this can give even well-done works of art a very unprofessional appearance.
So there you have it - a basic art method used by Professionals to help you be the best digital artist you can. Good luck with Your Art and Thanks for Reading!