In the Articles section of my firm's website I have "33 Things Your Business Planning Attorney Can Do For You".

Numbers 32 and 33 are "Document the duties, responsibilities and compensation of a company director or manager .... (and) company officer."

Unless you are a one- man shop, it can be very useful to have a written agreement between your corporation's directors (managers if you are an LLC).  Actually, even if you are a one-man shop it can be an added layer of protection so that a creditor, judge or IRS agent cannot later question whether you failed to fulfill or overstepped your duties to the corporation or limited liability company. 

Some things you might want to cover in this agreement are the minimum attendance requirements at director's meetings, participation in extra committees, attendance at outside events such as meetings with bankers or employee recognition days.  The director's compensation should be addressed as well.  Officers and directors have a fiduciary duty to avoid actions that hurt the company, such as competing with the company while on the company's payroll.  However, to prevent an officer and director from competing with the company immediately after leaving the company, a written agreement is needed.  The Laconic Law Blog posts a case in which this type of agreement gave the company the ammunition it needed to get damages from a director that chose to compete anyway. 

For officers of the corporation, you'll want to include duties specific to that position, such as the secretary's duty to take, disseminate and maintain minutes of meetings. 

How often should you revisit these agreements?  It's probably easiest to streamline this task by reviewing them at every annual meeting just before asking the newly elected mangers or officers to sign them.