* Frugality: deeply understanding the things that bring you satisfaction and skipping the rest
* Aggressively saving and investing money
* Building a mastery in personal finance
But becoming a millionaire isn't a math problem. Most people on here are capable of doing so many times over without any great career home run.
However, achieving such a milestone will require you to (on an ongoing basis) look deeply inwards, answer fundamental questions about yourself, and change your life.
At the risk of getting too preachy, most people haven't even figured out what they want and so they default to mainstream consumerism. Such people have little chance of becoming a millionaire (which requires sustained, long-term behavior changes) until they first sort themselves out.