The one thing that all 401(k) millionaires have in common, according to a Forbes' article "Nine Ways To Be A Millionaire In Retirement," it is saving at a much higher amount than others. Whatever your career path may be and whatever your earnings level is, start saving early.
While millennials often have a competing priority with paying student loan debt, it's still important to make sure some of your money is going into retirement.
Live like a college student. Even if you are making very little, you should "mind the gap" – that is, the gap between what you spend and what you earn. Make sure there is a gap and keep your expenses low. Try to live like a college student when you're earning your first salary. Maybe have Ramen noodles and hot dogs on at least some evenings.