[Katie_UPS]

First thing you do, is you advertise the team everywhere: have a robot open house and freshman orientation, put posters in the hall, if your school lets you-have the robot during lunch. Teachers on our team will normally do a quick shameless plug in their classes.
Once you're established enough, FLL teams at middle schools bring kids in. Demo-ing in the community also can bring attention and teammates.

Remind students that no prior skills are nessicary to join. Remind students (and especially parents) about all the scholarship opportunities.
Another big thing is roping your friends in, and convincing siblings, and siblings friends, and friend's siblings.

Have members talk about the team- why they like it and why people should join. Mention all the opportunities that FIRST has to offer... and that its not just about the robot. Artsy kids might really dig animation. The forensics kids might really like the marketing aspect (and presenting for chairman's). And so on and so forth.

Lastly, if you can people at a competition- official or otherwise- they can see how crazy and fun FIRST is.