What do millionaires know about credit cards that others don’t? That’s the question I was asked by NerdWallet. Check out my secret, as well as gems from others “in the know!”

My biggest purchase was $28,000 for a domain name. With a marketing background, I know how critical a great domain is, and I’ve been using credit like Nerd Cash-back since I was a teen. I learned about carrying balances the hard way in college, but got disciplined in the real world.

Sam Dogen retired early, boasting his biggest credit card purchase as $8,000 for bathroom tiling. He only uses credit cards with rewards he’ll really use, since it’s free money. In his younger days, he carried a balance from time to time, but committed to paying off his cards monthly before interest accrued.

For Lee Huffman, a finance manager, his biggest purchase was in the $9,000 range to buy a minivan. An advocate for using credit responsibly, he has a slew of cards with different rewards geared towards traveling.

For Doug Nordman, a retired military service member, his biggest purchase was almost $15,000 to install stamped concrete. He got rich “the old fashioned way” with smart investing. He also pays off his credit cards monthly. Read more about tips and tricks the wealthy abide by with credit cards.