What sort of power are you talking about? Physical power? Influence by way of position in society? Or the inner strength of someone who is very sure of who and what they are? That is a powerful character. There is a big difference.
What makes for physical power is exercise such as 'pumping iron,' the right type of diet, and the determination to built up one's muscular strength.
What makes for influencial power and position: planning, scheming, ambition and money.
What makes for the real inner power: exposure to life, dealing with it successfully, and continuing to make oneself a better person characterwise.