I am on the board of a community garden in Los Angeles. The most important factor for a successful community garden, I know this sounds cheesy, is people. You really need to be selective about the people that you let into the community garden. We have a waiting list for plots in our garden, and we require that people on the waiting list attend meetings and events before they get a plot. All too often, people are enthusiastic for a month. Then it gets hot, and they don't want to come out in the heat. The plot becomes run-down, and the plants start getting disease. This is really bad because pest insects take over the plot and then migrate as populations increase thus spreading disease. So it is easy to see how on bad plot can cause serious problems. This leads me to another tip, have no mercy. If someone isn't pulling his or her weight, by all means talk to them and let them know that they need to take better care of their plot, and do work for the garden in general, but if that doesn't work kick them out and hope fully replace them with someone that will really be a benifit to the garden. You can always grow cover crops in the empty plot. The last tip I will give you is to automate as much as possible. Working a large community garden is a lot of work, and if you can install drip irrigation and timers, it will make all of your lives a lot more enjoyable.