Don't open accounts just to have a better credit mix—it probably won't improve your credit score.

Unnecessary credit can harm your credit score in multiple ways, from creating too many hard inquiries on your credit report to tempting you to overspend and accumulate debt.

Opening a new credit card can increase your overall credit limit, but the act of applying for credit creates a hard inquiry on your credit report. Too many hard inquiries can negatively impact your credit score, though this effect will fade over time. Hard inquiries remain on your credit report for two years.

Keeping unused credit cards open—as long as they're not costing you money in annual fees—is a smart strategy, because closing an account may increase your credit utilization ratio. Owing the same amount but having fewer open accounts may lower your credit scores.