How to Be an Effective Independent Director
This post from Concrete Covina outlines some of the qualities which make for an effective independent director of a private company.
Summarizing the post, that is worth reading in its entirety: the director should be aware of the legal fundamentals of the position, particularly the duty of care and the duty of loyalty. The director should be motivated, proactive, and critically think about the company, its objectives and strategy. The director should add value appropriate to the company's stage of growth, be able and willing to serve as a Board committee member and be skilled at questioning.
This follow up post from Concrete Covina presents a grid as a meaningful quick method for reaching an opinion on the effectiveness of a director.