Make sure you have a focal point
Make your focal point off center
Use a full range of values
Overlap, overlap, overlap... have a foreground, middle ground and background
Crop! Decide on what's important and get rid of everything else
Never set objects or people on the bottom edge of the page or have them leaning on the sides of the page.(unless you are making a very strong emotional statement)
However, don't be afraid to have elements of type (if the word is understandable) or unimportant picture elements bleed off the page.
Use complementary colors in the shadows and dark areas (be subtle)
Don't use every color known to man in one piece of art (unless it's Fauve!)
Stand back from the monitor and LOOK.
Rotate your art until it's upside down and LOOK. Squint so the picture is fuzzy and LOOK- is there a full range of values from light to dark?
Is an unimportant area grabbing your eye first, and not the focal point? is the composition unbalanced (in a bad way) so that it looks off-kilter?
Is one color in an unimportant area grabbing your eye when it shouldn't be? Is the overall look bland and boring and in need of a pop of color?
Watch your edges, inappropriate hard edges of a cut and paste in digital art catch the eye and ruin the illusion of one integrated piece of art.
Don't force your pixels. If the image is too small to work, take another photo.