Take regular physical exercise: This will improve blood flow and oxygenation in the brain, and good general health promotes good mental health. (I can highly recommend Tai Chi for people of all ages and physical abilities. Many people also recommend yoga for similar reasons.)
Make sure you get enough sleep: Numerous studies have shown that lack of sleep can impair mental function in the areas of alertness, attention span, concentration, short-term and long-term memory, problem-solving, reaction time, visual and auditory acuity, and so on – i.e. pretty-much every area of mental function! Sleep – and especially the right mixture of deep and REM sleep – is vital for maintaining good mental health. (Note that both physical exercise (see above) and meditation (see below) can promote healthy, regular sleep patterns.)
Eat a healthy, balanced diet rich in ‘brain foods’: Your brain needs certain essential nutrients to perform at an optimum level, particularly with regard to neurotransmitter-precursors, electrolytes (i.e. salts) and fuel (primarily glucose). By eating a healthy, balanced diet rich in fruits, vegetables, vitamins, amino acids (especially from animal protein, e.g. chicken, dairy products and fish) and carbohydrates, you can maintain healthy brain function without having to buy expensive dietary supplements (some of which work, but many of which don’t).
If you are a smoker, STOP!: The various toxins in cigarette smoke can impair brain function in various ways, and have been linked to long-term cognitive decline.
Moderate your alcohol intake: Drinking alcohol in moderation, especially when combined with socializing, can be beneficial for brain function in various ways. But excessive alcohol consumption can impair brain function, both acutely and chronically, and may, in extreme circumstances, cause permanent cognitive impairment.
Learn meditation, e.g. Mindfulness: Meditation has numerous proven beneficial effects on brain function, such as improved concentration, attention span, creativity and alertness.
Try smart drugs, i.e. nootropics (if you can obtain them legally, of course) : These are clinically proven to boost mental function in the areas of concentration, alertness, memory, mental stamina and so on.
Regularly expose your brain to novel learning challenges, e.g. learning a new language, musical instrument, area of mathematics, etc.: This will promote mental plasticity and versatility. (And I genuinely believe that learning any area of mathematics, in particular, will probably boost your IQ by a few points; mathematics is an excellent mental workout, and exercises logical, numerical, visual and other forms of abstract reasoning, as well as improving concentration and mental stamina.)
Join a general knowledge quiz team: This will improve the brain’s ability for rapid recall of frequently random, unconnected and obscure pieces of information. It is also a very sociable pastime in the company of usually quite smart, interesting people.
Cultivate friendships with intelligent, educated people, and make a point of meeting up with them regularly for stimulating conversation on a diverse range of subjects: Through challenging intellectual discussion with intelligent people you will be exposed to novel ideas and fresh perspectives, and will learn the ways in which intelligent people think and express themselves, and these will often rub off on you.
Watch a diverse range of educational TV programmes, i.e. documentaries, factual films and current affairs programmes, and be more discriminating in your viewing generally: This will improve your general knowledge and expand your conversational repertoire (and thereby your social confidence), and you may discover new interests. (For an excellent online source of documentaries on numerous subjects, click here.)
Read at least one ‘serious’ book per week, and diversify your reading: This will improve your vocabulary and verbal intelligence, and increase your general knowledge and conversational repertoire.
Play a mentally challenging computer game: First-person shoot-em-ups and racing games are fine for honing your reaction time and exercising your trigger finger, but don’t offer much in the way of mental stimulation. Choose a game that involves learning complex rules and strategies, and demands a variety of mental skills. (I might be biased, but I think the game I play beats all others hands-down in this respect – EVE. It is quite simply the deepest, most complex and sophisticated computer game ever developed (and it runs on the most powerful gaming supercomputer on the internet, called Tranquility), as well as being visually breath-taking. “A game designed by geniuses for geniuses,” as a friend once described it. Go to Eve Online -> Media -> Videos, then watch the videos, and you’ll see what I mean!)
Use brain-training software that is proven to improve working memory and fluid intelligence: Most brain-training computer games do not produce the cognitive benefits that they claim, because they generally only train you to become better at playing that particular game; the improvements are not transferable to other types of brain-training games or translatable into improvements in general cognitive abilities. However, one brain-training game that has been proven to produce measurable improvements in general cognitive abilities, specifically in the areas of working memory and fluid intelligence, is Dual N-Back. Even better, the game is free to download and use.
Challenge self-limiting beliefs about your intelligence: Many smart people are smarter than they think they are , but they have never realized their true potential because of self-limiting beliefs about their supposed lack of intelligence, often implanted in them by overly demanding parents or overly critical (and often incompetent) school teachers. By a process of systematically questioning the factual basis of these beliefs, and coming to the realization that they are actually groundless, they can let go of them, and finally unleash their true potential. I know a girl who had such self-limiting beliefs, and worked as a humble office secretary, until one day she took the Mensa IQ test, and passed it. She now has a PhD in astrophysics, a rewarding career, and has never been happier. And the moral of that story is simple – dare to believe in yourself!
 This is known as the Dunning-Kruger Effect, where people of lower than average intelligence tend to over-estimate their intelligence, whereas people ofhigher than average intelligence tend to under-estimate their intelligence.