Sorry, haven't dealt with them. The best way to a good credit record is to consistently make payments on time. People promising a quick fix are either lying, advising something illegal, or both.
My suggestion:
First, pull your credit reports. You're entitled to one free from each major agency each year, through [1] below. From there, figure out what's bringing your credit down. If you purchase a report-and-score kit from somewhere like [2], they'll include an analysis of specific factors, both positive and negative.
Next, fix the mistakes. If there's flat-out wrong information on your record, like delinquent accounts that aren't yours, dispute them. If the creditor can't prove that their records are right, they have to remove the record. If there are accurate negative items, like unpaid collections, on your report, pay off any remaining balances. You may even be able to negotiate with the creditor: "If I pay this off right now, will you take it off of my credit report?" Everything on your report has to be true, but there's nothing saying that everything has to be on your report. Disputing accurate negative information, like some of these credit-fix workshops suggest, is ILLEGAL.
Finally, make a point of paying your debts on time, every time. Late payments mean late fees and higher interest rates, and those just keep you in the hole longer. If you want to learn more about consumer credit and how to build up your score, I highly recommend Fool.com [3], a financial site that's sorta affiliated with Yahoo! They have pages and pages of soild information, plus discussion boards (for a fee) where other users can answer specific questions or provide support.
Okay, sorry about the plug. But I love their site. Point is, there's no quick fix. It's a matter of self-control, and showing creditors that you're in charge of your finances.