In this feature, Dan Moyer shares some really great tips on how to improve your games visual design. He shows a lot of the material in his slide show which I have uploaded as a PDF (Visual Perception And You).
One of the things I really took from this is the use of lighting and gray scale in your design. One tip Dan shows is to convert your game to black and white and make sure that you can clearly see everything as you want them to be seen. Along similar lines, he discusses the use of lighting to illustrate important points for the player. If you want the player to be drawn to something, make those areas are lighter and brighter than the surrounding areas. This technique also helps for focus between the foreground and background.
The whole thing is well worth a watch and you should really look through the slide show while you do it. I promise that next time I will have a way better camera.
Follow Dan Moyer on Twitter: @Daniel_Moyer