Well I am a part of The FEDS up here in Michigan, and we pull in consistently about 45 members a year from our high school.
To do this though isn't an easy task; we have a rigorous recruitment process that goes down to the elementary schools. What my team has found successful is displaying to our school and community how strong of a program we have. Ask anyone who is familiar with the High School and they will mention about the robotics team and how well we did last year.
When we start advertising to the public, we start at the elementary schools. We do a robotics night at many of the elementary schools in order to spark their interest at a young age. Once they are in Middle School, they participate in a robotics program which is similar to FIRST. This inspires about 15 people per grade per school since it was started. In High school, after all of this exposure to Robotics has been shown, we as a team heavily advertise our team to our student body through announcements and in class demos. These videos and demonstrations are really what bring in those who have been interested in robotics, and even those who are not.
Along with these things, the students on the team constantly tell the younger kids that they know about how amazing robotics is. This is the real kicker. I can't tell you how much of our team joined because their friends were in it, which inspired them to do the same.
Although this works for my team, this may not work for others. I live in a community in which FIRST teams are in abundance (people from my city are on 5+ teams), and the coolness factor of them are publicized.
The best thing for you to do though is make a 2 minute commercial about your team. Display this commercial at the schools as a video announcement and forward it around to friends and school administrators. You will find that people like videos
~Alex