As others have said, it's all about advertising. They've posted great ideas about that here, so I'm not going to re-hash them. From what I've seen, the biggest reason kids don't get involved (especially freshmen) is the time commitment. It's not like soccer, football, baseball, basketball, or any other sport - this isn't something they've done before and thus know it's worth the time commitment. It's something that sounds like fun, but they just don't
know for sure that it will be.
So, your job is to first get them interested in learning more, and then get them hooked. Advertising will get them interested, getting them hooked takes a bit more. It takes getting them in a lab or shop and doing some actual work. It takes getting them interacting with the rest of the team and seeing for themselves how fun and stimulating robotics can be.
We started a "summer camp" for prospective students last year, and it seemed to work really well. We got a lot of incoming freshmen and sophomores to show up for a few hours a night for two weeks, and worked with them building Junior Theremins from this site:
http://www.madlab.org/kits.html
Those are great little projects that can be done quickly, get someone hooked, and give them something they can take home with them.
So, I recommend once you get a group of interested people, get them in the shop and interacting with your team before the season starts. Focus on something that requires a low time commitment but teaches them valuable skills while having fun.