Make your payments on time. Seriously.
The credit score, more specifically the FICO score is made up of 5 factors.
So let’s break down the first and largest of these factors, payment history. Your payment history makes up roughly 35 percent of your total credit score. So making debt payments on time is crucial to maintaining a strong credit score. But, don't let this freak you out, I don't mean to scare you! This doesn't mean, "Oh I forgot my car payment was due on the 10th and now it's the 15th and I'm in trouble." You may be charged a fee, you won’t be able to get out of that, if you're late a few days on your payment. But your credit won’t be affected by this late payment.
So, your credit score will only be impacted, if you're what they consider to be delinquent. This usually occurs when a payment is between 30 and 90 days late. The later, the more sever this impact can be. If you're late enough, you lender could actually just give up hope that you'll ever pay them back and sell your debt to a collections agency for pennies on the dollar. If this does occur, it will have the most dramatic impact to your credit score.
So this is why keeping track of all those payment due dates is one of the best things that you can do for your credit.