Never Stop Learning
Giving up your quest for knowledge is one of the worst traps that a person could fall into. Truly intelligent people stay humble, open-minded, and they hunger for more information. It doesn't matter what you're learning about as long as you're learning something new.
Think Creatively
When presented with a problem, try and think through all the possible ways you can navigate that problem. Seriously, think through absolutely every option and start making hypotheses on your own. By creating your own personal pathway to solve problems, you will ultimately become more intelligent and open-minded.