I have been asked to sit on a few Board of Directors (BOD) and Board of Advisors (BOA) in my time. Some were Not for Profits some were for private businesses.
Surprising the NFP boards were the most difficult to be on, they tend to be more social oriented less goal oriented. I have also found that some people like to get on these boards so that they can then exercise power that they might not have elsewhere in their lives. Or they join because they have their own agendas.
But the for profit businesses, there you see the impact of your work quickly. You are there to monitor and review the movements of the company and provide feedback, direction and guidance. There is the added benefit that you are typically compensated as well.
I have heard that some sign up with search firms to assist in finding appropriate Board positions for their skill sets. Also pink sheeted companies are an options as well.
But there is a risk with being on any BOD. As a Director you can be sued for the malfeasance of the company.
If you are on a BOD make sure you are covered by D&O coverage to a level you are comfortable with; because you would be personally liable for a judgement over the covered levels.