After seven years of working in the corporate world, one New York City-based twenty-something had a nest egg big enough to retire early. She goes by the pen name J.P. Livingston on her blog The Money Habit, where she discusses how she had financial independence on the mind from an early age. When she was only 12, she picked up a copy of the personal finance classic "Rich Dad, Poor Dad," which sparked her interest in saving and investing. When she realized she could save big by finishing college in three years, she jumped on the opportunity. Graduating a year early "really helped me financially," she tells CNBC Make It. "It saved me a year of tuition and gave me a year's worth of extra income, and between the two things, that's a $150,000 net worth swing."

After graduating, Livingston took a job at an investment firm, where the starting pay was $60,000, plus an end-of-year bonus that was almost the same amount. Total: just over $100,000. She received several raises in the years following, and by the end of her career, Livingston was making mid-six figures. After seven years of stashing more than 70 percent of her income, she built a nest egg of $2.25 million, 40 percent of which came from investing and 60 percent from saving. It was enough for her to quit and settle down at age 28. Today, she lives in New York City with her husband, who still works, and dog. They live off a modest $65,000 per year, which allows them to maintain a high savings rate. Livingston made a lot of money, but she also used a variety of strategies to save big over the years, such as tracking her spending and automating her savings, but it was one mental trick that made the biggest difference: Think about purchases in terms of cost per hour.