1) People who are actually intelligent know from experience that nobody wants to feel dumb, so it is not socially advantageous to brandish their intelligence, especially when their actions, vocabulary and reasoning can demonstrate it in abundance. They also know where they stand because they can compare with other people, and don't need to compensate for their insecurity with unnecessary bloviation.
2) Walk the walk, instead of talking the talk. There is no lack of wastrels or C-list celebrities who tout their Mensa IQ etc. without being able to accomplish anything worthwhile with their lives. As Hawkings puts it, "people who boast about their IQ are losers." Meritocracy is about concrete skills and results, not hot air.
3) As Terry Tao puts it, kids who are praised for their intelligence come to associate success with intelligence and, likewise, failure with the lack thereof. They might try something new and fail because they lack experience, but mistakenly believe they don't have the innate talent and give up sooner than they should. It is better to praise them for the work they put in to accomplish the results, than to orient their lives around the myth of the chosen one, which is a harmful fantasy trope.
4) This board might contain a lot of cancerous shitposting about intelligence, but do not for a single moment confuse it with real life. Intelligence, misused or wasted, can easily lead to bitterness, tunnel vision, self-deception, social isolation or a superiority-inferiority complex. So don't for a moment think you automatically have it made just because you notice you have a brighter bulb in your noggin than your fellows.