Most people have some sort of debt looming over their heads, and while the select few that do not should feel proud of their accomplishment, they may not be doing anything to help their credit scores. Credit bureaus take into account the various lines of credit one is able to juggle, and those who use credit responsibly are better off than those who do not use it at all.
If you are one of the lucky individuals that are without debt, consider opening a major credit card to put small purchases on. If paid off monthly, you can slowly raise your credit score to a better level. The company that determines your FICO score likes to see a mix of revolving credit (credit card) and installment credit (car loan, mortgage, student loan). This helps them to know that you can handle these responsibilities, thus making you a more likely candidate for loans in the future.
The key is not to accrue unnecessary debt. Simply use the card for a small purchase, such as a movie ticket, and pay off the bill in full as soon as the statement comes in the mail.