If you still have the very first credit card you opened in high school or college, don't close it. Even if it's fallen into disuse, it could still be helping your credit score.

When you close an account, you're shrinking your available credit, which can harm your credit utilization ratio. "Now all of the sudden, it looks like you're using more of the available credit that you have remaining," Greg McBride, chief financial analyst at Bankrate.com, tells CNBC Make It.

The more credit available to you, the lower your credit utilization ratio will be. This is helpful because it's ideal to use only 10 to 30 percent of your available credit at any given time.

"The 10 percent threshold is the point at which it's beneficial to your credit score," McBride says. "Between 10 and 30 percent it's neutral, and it's only when your balance is above 30 percent of your credit line that it actually works against your score."