The single best investment you can make is something most people wouldn't guess: It's investing in yourself. Investing in yourself can make you a millionaire – without having to worry about what the stock market is doing.

How do you invest in yourself? It's S.I.M.P.L.E.:

S – Strive for Perfect Credit
As I explain in my book, 'Perfect Credit: 7 Steps to a Great Credit Rating,' having pristine credit is often better than cash in the bank. With A-1 credit, you get the best rates on car loans, credit cards and mortgages. You also position yourself for good jobs and promotions, since employers are increasingly conducting employment-related credit checks when deciding whom to hire and promote. For loans and career opportunities, having outstanding credit will help you save or earn hundreds of thousands of dollars during your life.

I – Improve your education
Consider earning a degree if you don't have one, because U.S. Labor Department statistics show that college graduates earn 62% more money than high school graduates. Over a lifetime, that translates into a $1 million earnings gap.

M – Make it a lifelong endeavor to increase your knowledge, training and skills
Constantly upgrading your skills makes you more marketable and attractive to employers. Focus on a variety of areas: communication skills, job skills, and technical skills, along with leadership and management skills, too.
P – Put money in your own business
Entrepreneurs are far wealthier than most corporate employees. In fact, 6 out of 10 millionaires in the U.S. are self-employed. Check out The Money Coach's Guide to Your First Million for success strategies of the wealthy.

L – Learn to give back
You reap what you sow. But don't just give to receive. Give in order to help others and you'll find yourself rewarded in many ways. You can give money, but you can also donate your time, talent and resources.

E – Earn more by creating stability in your personal and professional life
Constantly "trading up" is expensive – whether you're talking cars or relationships. Don't lease new cars every year or two; that's wasting money. And don't let your love life become a revolving door. Divorce is expensive, and often takes two to five years to recover from financially. Instead, maintain stability. Besides, banks and employers alike prefer to see longevity. So unless you're unhappy or unfulfilled, stay with the same company instead of job-hopping every year. Also, keep the same address/phone/email instead of making frequent moves. That looks bad.

Taking these actions above to invest in yourself is a "sure thing" – far less risky than gambling on Wall Street.

Follow me on Twitter for more financial tips. Also, share your thoughts on this topic: What have you done to invest in yourself?

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Lynnette Khalfani-Cox, an award-winning financial news journalist and former Wall Street Journal reporter for CNBC, has been featured in the Washington Post, USA Today, and the New York Times, as well as magazines ranging from Essence and Redbook to Black Enterprise and Smart Money. Check out her New York Times best seller 'Zero Debt: The Ultimate Guide to Financial Freedom.'