1. If they ask for money up front. It is illegal for a credit repair
company to ask for money up front according to the FTC
(http://www.ftc.gov/os/statutes/croa/croa.htm).
2. If they promise to wipe your credit clean. No credit repair company
can truly guarantee performance of any kind.
3. If they ever ask you to establish a new identity. Doing this will
get you in serious legal trouble.
There are some credit repair companies out there though, I've used NCCS
( http://www.nationalccs.com ) in the past and they were very helpful
in removing some erroneus items from my credit report. The best part is
that they only charge you for items they were able to remove or correct
on your report.