We had very similar problems... our solution was to try and make Leaders out of every one of them. We did a leadership workshop, selected team leaders, and got students to lead subteams, demos, fundraisers, etc. Im pretty happy to say that nearly every one of our students lead some activity.
It wasnt an easy year, we had to make requirements to make most of this work, but now a lot of our students jump in and help out when needed, many of them have much more thought to put into conversations, and the ideas coming from them are much more realistic and well thought through. We also had to be willing to fail. We had two student lead fundraisers that raised less than $10 each. But they learned from them.
This is a really quick summary, you can see some of my other posts or look through the leadership material I gave out at the CMP conference:
http://www.penfieldrobotics.com/Leadership/
Hope this helps!