Forming a helpful board of directors for your start-up can be a puzzling task. First you have to check legal requirements in the country the company is registered in. Germany has a two-board system with well regulated requirements for the roles of directors (you may check the GmbHG and AktG with Google). The UK and the US states mostly do not specify the roles and requirements for a board member at all. So it's up to your creativity and negotiation skill to find people that a) have strong credibility b) can leverage your business and c) are no cost killer for your business, In search for more in information Google lead me to this helpful site with a good overview what directors can do for you:

Area	Advisory or Regular Member	Active Board Member	Assigned Projects
Finance	Review financial plans, statements Identify/contact sources	Executive staff planning meetings Review bank and leasing terms and advise; attend key meetings	Drive and guide planning process Help conduct negotiations for bank and leasing company
Financing	Financing advice and consent	Help prepare presentations and materials	Find money sources, strategic partnerships, private investors and venture capital
Human Resources	Final interviews	Consult on HR policies Interview candidates	Help design and document performance and/or salary review process Primary recruiting agent
General Management	Review/advise Advise on executive staff personnel and performance issues	Specify process needs Review strategies, define alternatives and suggest choice criteria Business plans; detailed review and revisions Discuss, investigate and make recommendations on staff issues	Design/document process Research/propose strategic plan Research and draft business plan Serve as interim or acting VP or CEO? where special circumstances require
Marketing	Review/advise Make contacts/introductions	Help review/select PR and/or advertising agencies Review product/promotional plans and advise Assist as high level spokesman for key activities (e.g., IPO, major introductions)	Product positioning Plan and/or conduct PR and/or merchandising programs
Manufacturing	Advise/assist in prototype manufacturing Review manufacturing philosophy	Participate in selection of vendors/high volume production	Introduce/help select off-shore manufacturers /vendors
Negotiations		Review terms	Define/document terms and/or conduct negotiations
Sales	Introduce potential customers to company	Attend sales presentations/assist sales staff	Help penetrate/close key accounts
Technical	Interface/advise on research and development projects	Participate in design reviews and strategic planning positioning meetings	Guide project team on a specific technical objective/schedule