If you are new to being a race director, you will find this resource compiled by Playmakers, a Lansing, MI, running store is a huge help. Even if you are a wise and experienced director, it never hurts to double check yourself.
To their list I would add one other important point, have an emergency plan — what to do if there is a major health issue, if threatening weather suddenly arrives, etc. Having a clear plan of who will do what in that situation is very helpful. It is very difficult to direct emergency services, or get competitors to safety if you have not thought through possible problems in advance.