Determine what the listserv will be used for.  Is it only open to a select group of invited participants (which requires someone to manually subscribe and unsubscribe participants).  Is it open to everyone, in which case an automated subscribe and unsubscribe function is very helpful.  Will it automatically relay messages to everyone, or will someone moderate and approve the messages first?  Will you send messages and replies one at a time, journal everything into a single communication of a daily or other frequency, or both?

Also, be sure you understand all the responsibilities and potential liabilities in operating a listserv.  As an example, antitrust laws can prevent people from comparing prices or fees on a listserv.  What type of content can, and will, you allow, and how will you enforce that?  How will you handle the inevitable spam that will hit your listserv?