Take Care of Yourself
Eating a balanced diet and exercising can increase your IQ. A study conducted in Sweden showed that people who exercise have higher IQs than people who don't. Exercise stimulates brain cell growth and provides needed oxygen to the brain. Individuals who are fit in high school are more likely to go to college than those who aren't. Taking care of the body is an important part of taking care of the brain. 

Another way to improve both your mental health and IQ is to develop a meditation practice. Meditation decreases stress, stimulates brain activity, and over time can increase IQ scores.

Play Games
Playing logic or strategy games has been shown to increase reasoning abilities. Games like chess and Sudoku help the brain learn how to make connections and solve problems. When you master one game, find a new one to play. The brain doesn't work as hard once you become skilled at something, so continually find new games to play that challenge you mentally and improve your reasoning skills.

Playing video games has also been shown to improve intelligence. If you already play video games, pick a game that's outside of your usual selections to broaden your skills and stimulate the brain. Pay attention to what is going on in your game, rather than just mindlessly running through the story. Logic video games, like Tetris, have shown to be effective at increasing brain activity. 

Keep Learning
Learning new skills and information will keep the brain stimulated and help increase IQ. Find a hobby that you enjoy and one that challenges your capabilities. When you become proficient at your chosen skills and hobbies, find new ways to do them. If they ever become routine, then they lose their challenge and the learning stops.

Be observant. Pay attention to the details of your surroundings. Part of learning is paying attention and observing what is going on around you.

Take a class.
If there's a subject you've always been interested in, explore that subject by signing up for an online class such as the ones offered by StudySelect. Online classes offer a wide range of subjects and the ability to study from home, so commuting doesn't need to take up time or money. From physics to creative writing, you can choose something that interests you and will keep your brain stimulated.

Engage with others. The more people you meet in person and online, the more ideas you'll be exposed to and the more you'll learn. Discuss with friends topics that interest you and share what you have learned with them.