Some tips:
1) Mirror your image often. Every few minutes, flip (mirror) your digital canvas horizontally. If you are working traditionally, look at your picture in a mirror, or hold your paper up to the light backwards. You will see errors that are not otherwise visible. Often your picture will look skewed (pushed diagonally).
2) Clean up your lines. Unless you plan to have a detailed, textured image, lines need to be smooth and without excess strokes. There are two main purposes of lines - to define shape (
http://www.icefox-studios.net/centaur_ink.gif), and to add texture (
http://pratt.edu/~mcorrier/sketch/warface.jpg). If you\'re aiming for a sketchy look, then by all means ignore this, but if you would like to have a polished, finished image, then erase the scribbles you don\'t need, and vary your line width. It prevents even simple line art from looking flat. To do that, you\'re going to have to think of a light source, a direction the light is shining from, and then figure out where the image is going to be darker, and make your lines thicker there.
3) If your image is more sketchy/texturized than pure line art, coloring it in any paint program will only take away from it. So unless you ink your picture, or at least erase extra pencil lines and make the others darker, you better leave it without color. Otherwise, the next step is either separating your line art from your background, or painting over it and letting your lines show through. This means no white spots where the lines and color meet. Again, think of a light source, and concentrate on your lights and darks to avoid having a flat picture. Also, don\'t blur colors all the time. Avoid soft-edge brushes and pick some hard ones. If you smudge too much, it will look washed out.
4) Color choice. Think of natural colors, and complementary colors. Study the color wheel. For example, don\'t color the body of a creature blue then have it quickly fade to red. While nature can have a lot of bright colors and surprises, they are always well-matched. Think of where your creature will live. In the desert? Choose warm colors, such as yellows, reds, oranges, browns, etc. In the forest? Choose yellows, greens, and browns, with perhaps red for decorative elements if its in the tropics. In the ocean? Stick to cool colors, such as violets and blues. It\'ll make a difference.
5) To make it really well-done, don\'t paint on a white background! The color and value surrounding any area you are working on affects your perception of color and value. A red background makes your subject look green, a light background makes your subject appear dark, etc. Rather than a white canvas, start with a neutral tone, like a grey or beige. If your picture is going to have a lot of red in it, you could start with a medium red. Once you have a background color in place, lay down flat areas of color for each object in your picture. By defining the correct colors as early as possible, you can see how the colors will interact when the picture is finished.
6) Do not create lights and shadowed areas using white and black. This creates a very dull and artificial appearance and assumes that the light source is pure white. Most light is colored. The highlights on an object will be tinted with the color of the light, and the shadows will be tinted with the complementary or opposite color. Afternoon sunlight, for example is yellow, and will cast blue-purple shadows. Fluorescent light is blue-green, which casts a slightly orange shadow.
Think that\'s all for now, hope it helps :)
