If you have the resources I would also strongly recommend presenting at local middle schools. Being a feeder school, this works especially well for us. Knowing that almost all of the local middle school's promoting class will probably end up here, we can work to our advantage.
As for keeping people interested, see if there are any off season events in between September and January your team can participate in. Here we have Battle at the Border, which comes very close to the real experience of a regional. It really encourages new students when we tell them that it isn't really the same level of energy at a regional.
If you can't go to off season events, try miniature build challenges. Give them a game to design a robot for, and the review their ideas. This year we had semi-veteran members mentor groups of newcomers while they designed robots for Lunacy. Any game would work, but we picked Lunacy because it was our mentor's favorite.
