We hold a summer camp for incoming freshmen that seems to work quite well. We have 13 freshmen attending this year, out of a class of about 80, which is typical. So a smaller school, yet we have 25-30 students every year.
When talking with the incoming freshmen, you need to emphasize your training program. No experience needed, learn how to build/wire/program a robot. Signing up for our summer camp doesn't commit you for the entire season, it's just 2 weeks, and a smaller commitment is much easier to make. Get them in the door, and then its your job to keep them