[dubious elise]

Simply asking around in your classes (especially science, math, and tech ed) can generate an awful lot of interest. Remember, not every girl does downhill ski racing or basketball all winter long!
Also, as Amy and a few others mentioned, demonstrations are an excellent idea. I first heard about robotics when I was in 5th grade - being from a small parochial school, seeing a girl that had graduated a few years before, attended the public high school, and had become a leader on the robotics team was very exciting for both my friend Molly (our electrical captain) and myself.
Once you get girls on the team, tell them exactly what our only female mentor told me: "When something looks neat, just elbow the guys out of the way and give it a hand yourself." They need to know that, just because there may be way more guys than girls, they too can make a huge difference if they are persistent. It always seems intimidating at first, make sure that if you have a weekly team meeting, it is open to everyone and be sure to encourage the girls that do show up.
From what I recall, there have always been at least 3 girls on the team, this year, we were blessed to have 7, but that doesn't mean that there isn't any room for more!