'Shark Tank' investor Daymond John shares the best business advice he ever got

"Shark Tank" investor and entrepreneur Daymond John wanted to run his own business for as long as he can remember.

"I never knew anything other than wanting to be an entrepreneur," the New York native told Business Insider.

In fact, his entrepreneurial career started in grade school. He would scrape the paint off pencils and customize them with the customer's name for a fee — his market was the pretty girls in his first-grade class.

Since then he's made a fortune launching the clothing line FUBU ("For Us By Us"), and more recently has established himself as a savvy investor on "Shark Tank."

When CNBC asked John about the best piece of business advice he's received throughout his lucrative career, one moment came to mind.

"It was from an older salesman I knew named Hal, when I first came into the fashion business on a big level," John recalled.

"I remember him saying, 'You have to save your money in this business because you never know — at one point or another, you're going to have to bring it back and invest it back into the business.' And he was right about that."

It's one of the reasons John continued his full-time job waiting tables even after FUBU started to take off.

"Even though I had placed our product in the hottest music videos out there, I was still working full time at Red Lobster," John told Tim Ferriss. "To the public, FUBU was a huge company. Little did they know that I was still serving them shrimp and biscuits!"

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank."