• Develop a habit to solve math problems. Pick up a quant book on GMAT, GRE, SAT etc.
• Solve puzzles like sudoku, 2048, doors etc.,
• Read a lot. Science, History, Humanities
• Watch/read investigative material (Sherlock Holmes)
• If you can, learn to code at least one programming language
• Subscribe to one of the Science channels to keep up with the latest in Science and technology. I would recommend I fucking love science(IFLS) page on facebook
• Keep a smart company. Remember your IQ is the average of 5 closest people you hangout with
• Take an IQ test, once a year at least

And last but not the least, stay hungry, stay foolish.