My husband and I have coached a team for three years, and it's a great program.  There's a lot of good advice above.  A few things that we've learned:
- We prefer smaller teams.  Aim for 5-7 kids who want to be there.  With 9-10 kids, it gets chaotic and one of the coaches ends up doing nothing but babysitting, keeping a few kids out of hair of the kids who are actually doing something.
- A mix of younger and older can work very well, but I agree with Kerry that the older ones only have so much tolerance.  One year, we had to diplomatically suggest that one little guy wasn't ready.  He was a great kid, but he was driving the other kids crazy.  Other young ones have worked out just fine.
- We like to have two meetings every week--one after-school meeting dedicated to the Project and one weekend meeting dedicated to the Robot Game.
- If possible, find a place where you can leave the table set up.
- We like to start the season with a build-the-robot-game-models session.  Everyone can participate and it's exciting to see the models and the table come together.  After the meeting, double-check the models to make sure that they're built and working correctly.  
- When we started, the Project was a pain, an afterthought.  However, if the kids choose the right topic, it can be as much or more fun than the Robot Game. 
It's a lot of work, but it's definitely worthwhile.  Have fun!