'Shark Tank' investor Barbara Corcoran reveals the productivity trick every entrepreneur should use

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"Shark Tank" investor Barbara Corcoran.

Since Barbara Corcoran made her fortune building and then selling one of New York's premier real-estate firms, The Corcoran Group, she has been a full-time investor on the popular reality show "Shark Tank."

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In the past six seasons, Corcoran has made deals with about 30 of the entrepreneurs who have appeared on the show seeking investments in their small businesses.

Business Insider recently caught up with her at this year's Canon Expo New York, where she was promoting Canon's Maxify line of printers, and asked her to share her No. 1 productivity tip for entrepreneurs and small-business owners.

She said it came down to a fundamental rule: When setting priorities for your company, begin by repeating what has worked in the past before you try anything new.

It may sound simple, but Corcoran said it's far from common sense. She said most entrepreneurs she's worked with hadn't sat down to figure out exactly what was working and not working in their businesses. Without that understanding, entrepreneurs can waste tons of time focusing on the wrong things.

Corcoran said after doing her due diligence on a company she's invested in, she sits down with the owners for an initial business meeting. She said: "The first thing I say to them is, 'Where has your business come from so far?' They never know!"

She gave a hypothetical example: She asks an entrepreneur how he made a deal with a client who accounted for 20% of his business. After some pushing, the entrepreneur remembers that it was a referral he received after giving a speech at an industry event.

Corcoran then asks how often he gives speeches like this, and the entrepreneur says it was a one-time thing. Before this easy line of questioning, the entrepreneur hadn't realized how much speaking at these kinds of events could benefit his business.

"So I start making a list of what's worked before and have them focus on that before I allow them to do something else," she said. She also recommends pursuing these tried paths to success before spending new capital on riskier investments, such as hiring a public-relations manager.

"I don't let them spend any time on anything that doesn't directly result in a sale," she said. "Because that's what you need when you're a small business."

If you want to grow but you're prioritizing the wrong thing, "you're just not gonna do it," Corcoran said.