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There is not easy answer to that question. The Main Street Program at the national level promotes a concept of downtown and neighborhood commercial district revitalization based on the four themes of historic preservation, organization, ecoomic restructuring, and promotion. Its success lies in that is is comprehensive. Too many efforts fail because they just focus on ne element. The reason I can't give you a good answer is that the Main Street Program is implemented at the state level, and is different in every state. In Wisconsin it is competitive, and only 1-3 communities are accepted every year. In Illinois, if you want to become a Main Street community, all you have to do is say "me too." In Wisconsin, all they receive is technical training. Other states provide grant funds. Your best bet may be to identify states that are considered good examples, and gather information about them.
I went to their conference in 2001 in Indianapolis, and really learned the ins and outs. Also at the time I was doing a report on the Mississippi Main Street Program, they have a great one. I met with the state organizer, and the coordinator for Natchez and Port Gibson programs.
Let me know if the info I gathered would help.
Also I am sure the checked the main web site, it is full of information on the organizational structure.