Read up on how credit scores are determined then build a plan to favor those conditions. For example, a large part of credit score is utilization ratio. The amount of used credit to available. I increased my limits on my cards, and actually got several new ones. While the application for new cards takes a small initial hit to your score, it will quickly rebound if you are using a very small amount of credit, and paying it in full every month. Try to keep utilization below 20%. Obviously, never miss a payment. Having different types of credit helps. ex, A mortgage, a car (or rv) loan, credit cards, etc.)
I apply every single expense i have, excluding rent, to my credit cards.