Be trustworthy. If they tell you a secret, make sure to keep it. Never tell someone a secret your friend wanted you to keep. Don't even leak it out to someone you know you can trust. A secret is a secret.
Spend time together. Hang out on the weekends or plan activities together every now and then, do some homework together, and chat during break at school. You don't have to live in each others' pockets, but make sure you spend some quality time together with your best friend to make the friendship grow and become stronger.