Here are 9 tips from Ted Kikoler for Making Graphics Sell:
Zoom in on the product
Use large-as-possible photos and illustrations
Crop them to cut away unnecessary material
Show the product in use—by a human being, if possible
Control eye flow
Most readers naturally shift their attention from dark to light areas, and from large to small objects
The eye focuses on things that look out of place
Use copy captions and call-outs with graphic elements
Use “handwritten” copy—but discretely
Script copy, unless overused, commands attention. Employ it to stress special thoughts or introduce long copy areas.
Break up large areas into smaller ones
To your reader, small copy or graphics areas look easy to digest
Use different graphics styles for different sections of your mailing
Otherwise, your reader may grow bored
Don’t strain for novelty
Use colors, layouts, and other design elements familiar to the reader
Use design appropriate to the product
Masculine products should look masculine, and vice versa
Hot colors produce hot responses
Use bright, hot colors (such as red) on your reply device
Keep your graphics simple
Make it easy for the eye to move from element to element