Check out this just released list of top 10 reasons that security clearance requests are rejected. You may have thought drug use or credit issues were to blame, but it's not nearly as juicy as that. It's mundane, simple mistakes such as missing information, lack of fingerprint cards or not providing information for relatives who were born outside the U.S.
Don't be part of the statistics. Work closely with your security officer and expedite the process by double-checking that all your information is correct. And make sure your fingerprints are properly rolled.