Before Grant Cardone built five businesses and a multi-million-dollar fortune, he was deep in debt and addicted to drugs.
It was advice from his mom that ultimately helped him dig himself out of the hole.
"The best business advice I've ever been given was from my mother, who was never actually in business," the self-made millionaire tells CNBC. "She said, 'The best investment you will ever make is in yourself. It's a-no lose deal. It will always give you a return. Nobody can take it from you. It's yours.'"
Struggling to make ends meet with his job at a car dealership, 25-year-old Cardone put his mom's advice into effect.
"Even though I hated the job, I decided I would throw myself into my sales job 100 percent," he writes of his younger self in his new book, "Be Obsessed Or Be Average."
He watched sales training videos while he ate breakfast, listened to self-improvement tapes during his drive to work, and was often the first employee to show up and the last to leave.
"I committed to learning everything I could about sales and the automobile industry," he writes. "In the first year I probably spent seven hundred hours just improving myself as a salesperson."
Cardone started to see results. He went from making $3,000 a month to $6,000 a month in his commission-based job.
"I remember that $3,000 leap better than I remember the first million I made because it was probably more important to me," he tells CNBC. "It changed my confidence. It changed my belief in me, and it also changed how I considered and defined my own potential."
By 30, the entrepreneur had notched his first million.
Today, the 58-year-old still follows his mom's advice, he tells CNBC: "Every time I read a book — every time I go to a conference — I walk away with something that nobody can grab from me. Competition can't take it from me, it doesn't cost me any more, and I don't get taxed on it — the government can't tax my improvement."