Advisors are an important piece of any project. They provide guidance as you develop content and offer advice on the best ways to reach your target audience. They are your experts. Advisors can be such a valuable resource for you that it is important to think carefully about the types of people you want for advisors and how you want to use them.
Some advisory boards, convened primarily for the benefit of funders, are made up of "big names." Their names can help open doors for you, but often they don't have the time to commit to your project beyond giving it their stamp of approval. If you need to have such advisors for funding purposes, think about balancing them with people you know can give you the time you need to give real, constructive feedback.
The best projects have advisory boards that span all the pieces, from planning to program outreach. Increasingly, funders require common advisory boards. Having one advisory board ensures that the different elements will be integrated and consistent. You will, however, very rapidly burn out your advisors if you require them to review every element of a project. Therefore, the best strategy is to establish a central advisory board and then to spread responsibility across the board depending on the individual advisors' skills, interests, and specialties. You may want to have one key advisor responsible for content across the project, but this person should be compensated at a much higher rate than your other advisors.
You might also consider having a national advisory board, which includes individuals representing both your project's content area and the target audience(s) -- essentially a joint production/outreach board. You could then have an additional local board made up of individuals in the trenches, people who can give a more practical review of your materials and activities and who can commit to piloting parts of your project.
As with all partners, the key to working with advisors is to be clear about expectations. How much time will you need them to spend? Will there be meetings? Will you cover travel expenses? What volume of material will they be expected to review? When? And on what kind of turnaround schedule? What will be their compensation? Will it be an honorarium or an hourly rate? Again, as with partners, the sooner you can bring them on, the better. When you talk to potential advisors about your project, spend enough time hearing their opinions before you commit to them; this way, you can ensure that their input will be useful. Advisors figure out pretty quickly when their advice is being ignored routinely. This isn't a good use of their time or yours.