[JakeGallagher]

We get about 15-20 real members that will put forward the effort needed to do well on your team. In the past few years, we have had lots of people join the team in the first couple of meetings, but then drop out after a few weeks. One of the main reasons is because of the stress we place on fundraising. Another reason is because we require a certain amount of time to be put into the meetings and build time.
Our team has a rule that each member must log at least an attempt at a certain amount of fundraising; if they don't the member won't be able to attend the competitions. We have sports players that can't help raise money as much in the fall, but they usually make up for that in the winter. Like already stated, it IS a pay-to-play game and if someone doesn't want to put forth the time needed to get money for the team, we take that as a message that they don't want to really help the team at all.
Another rule that our team has is that each member must log a number of hours equal to at least half of the time that they have been on the team, or they can't go to the competitions. The reason this rule was implemented was because one year, we had a member that never showed up to any of the meetings, fundraising events, or build sessions--and still went to the competition. Worse than that, he decided that he was the one who was going to drive and made a big fuss about it. Aaanyways... We don't "cut" people off the team if they can't show up for meetings though. If the member can make a concerted effort to get to the meetings, and helps in any way they can, we still let them go.
What else...? Oh yeah. We have one last rule that limits the number of members to our team. It's a school-wide rule, but we enforce it much more strictly than the sports teams do. It's the grade rule. Each member of every team at our school must be passing three of their four classes to be on a team, and they must pass every class to attend a team function (such as a competition). Our FIRST team has had to take key members (last year, for example--our programmer, who had a 67, three points under passing) off the roster of people attending the competitions because of grades.
I don't know how much this will help you in your weeding/selection progress, but it seems to make it easy for my team to keep small numbers...whether that's good or bad. :|

$.02