Having the discipline to continue to pay your bills on time gives the banks more trust. They will then be able to lend more money to you.
If you open up a new card, you will have the opportunity to have more credit afforded to you, making a lower utilization of your credit.
Becoming an authorized user on another person’s account will help you increase your score. It is like someone co-signing for you on loan.
If you have some unused old credit cards that do not have a fee, do not close them. That affects your credit history.
Make sure to keep your credit balance below 30%. If you use too much, the creditors will bring down your score due to the risk involved.