1) last year we held an all-day demo at our school because the drivers had just graduated (so they could miss classes). It got about 50 kids signed up. maybe 10 of them stuck with the team, but it still proved to work.
2) do as many fundraisers during the summer so that fundraising during the year isn't so harsh.
3) as stated: go to off-season competitions
4) have your parents talk to their friends if they have kids who would be elligible to be on your team. Maybe they can get them interested. They might even want to talk to you about how un FIRST really is.
5) Talk to your friends who might be underclassmen. If you keep mentioning Robotics, maybe they'll get the message and sign up.
As for getting mentors and other people invovled...
you can ask your parents to come if they are engineers or if they know any that would work too. Make sure they understand the game before the event starts so they aren't completely lost (which results in being bored).
when you try to get sponsors, introduce yourself as someone from your team. You might want to wear your uniform to show pride. Tell them the deal. Theyll listen. Tell them how you spread the word of science and technology.