The key is repeated branding. The person sees the billboard so many times it becomes branded to them. Just like the McDonalds golden arches. Research shows that a person must see or read an advertisement 6 times person they become aware of it.
Creating a billboard: Start with consulting a local billboard advertising firm. They will have good information on demographics and traffic of the area. That way you can match your product's targeted customer to the best location.
As for creative content: Again, talk with the billboard consultant. Word of advise...less is more. You need to get your message across in 3 words or less...including phone number (or website).
Images are key. Remember, billboard readers have only a couple of seconds to read the message.