Ten words of copy is ideal, with no more than five words in the headline.
The message is more important than the details. The message is the word, phrase or image that engages the viewer.
Details are things like phone number, web-site, and address. Use as few details as possible as the more you have, the fewer the viewer can remember.
When deciding on an image, choose one that generates the strongest emotion related to your product or service. For example, an image of people having fun creates more emotion than a picture of a soda can. Consider what the viewer is really buying; with soda they are not buying brown, sticky liquid, they are buying refreshment and an experience.
Images need to be high quality and resolution.
Images are greatly increased in size and will pixelate if the resolution is too low.
The best billboard designs are simple, invoke emotion and often use humor. It is important to remember that billboard design is completely different from other advertising mediums and is best used for branding purposes. You cannot simply put up a billboard and expect the phone to start ringing; not everyone that sees your ad will need your service at that specific time, but when they do your company will be at the top of their mind.
Think about soda: do you buy a soda every time you see an ad for one? Probably not. But chances are good that when you get thirsty, you will crave one!
IDEAS TO ADD TO DESIGN PAGE:
Gallery of Board Designs for various industries
Better visuals to explain text above
Divide by design strategy and technical requirements