Things to fill backgrounds:

• A component already used in the work, but enlarged and faded/blured
• Noise. Choose a color and noise it up a bit to give it texture. Blured noise which is then modified to reduce the color variance can make a very nice background
• Texture. Apply one of your painting programs fancy textures to either the background layer or the whole work
• Whore the filters. Use all those stupid filters to make a fancy background. Just make sure that you reduce the contrast of the background so that the forground elements stand out
• Of course, none of these can compare to actually generating a unique, fitting background for the work