Some links passed to me by readers:
Security Clearance Lawyers to help you through the process.
The Polygraph Examination: Forewarned is Forearmed
So far, being helpful and polite is important, too. My security clearance interviewer was pretty ticked off by the previous interviewee--who was rude and demanding. Also, dress nicely (you don't need a suit, but you should take it seriously).
Either way, best of luck!