Money Magazine has an article on four steps you can take to improve your credit.
The last section is the most important, how to clean up your credit. The first three sections deal with obtaining a credit report, making sure only "good" credit shows up on your credit report, and protecting your identity.
Everyone that doesn't understand what actually goes into a credit report needs to read this article. It's a short summary of good credit habits that will allow anyone to repair their credit and maintain a good credit score. If yours is below 700, or worse you don't know what it is, check out the article and follow their advice.