Serial entrepreneur Jennifer Fleiss knows what it's like to face rejection.

Founding the clothing rental startup Rent the Runway in 2009 alongside Jennifer Hyman, the then-business school friends secured a meeting with their dream mentor, fashion designer Diane von Furstenberg, only to be told their business idea wouldn't work. 

"She hated the concept, and that was incredibly demoralizing," Fleiss told Beatrice Dixon, co-founder and CEO of the Honey Pot Company, during an Inc. Your Next Move streaming event Wednesday. But that moment also taught Fleiss a valuable lesson about dealing with noes--and the importance of a supportive co-founder.

Fleiss said Hyman's response was: " 'Are you kidding me? She just told us all the ways that we need to evolve our sales pitch and pivot the business and tweak things.' " The early hiccup taught her to maintain a resilient mindset and keep an open mind about strategy.

The team later pivoted to a subscription model that allows shoppers to rent up to 16 items per month at a flat fee. The company was valued at $1 billion in 2019 before going public last month. Fleiss went on to become the co-founder and CEO of Jetblack, a Walmart members-only concierge-shopping service. This year, she took on a venture partner role at Volition Capital.

Here are the key lessons Fleiss said any business owner should know.

Build a strong network

While learning supply-chain and logistic skills on the job with Rent the Runway, Fleiss created an easy email trick to build a supportive group of colleagues to help answer questions on the fly. She created a mass email contact list of other founders or experts who could answer questions, give feedback, or brainstorm ideas. Sometimes, that meant just asking a question about an increase in shipping rates or how to handle a key employee's quitting.

"It might be 50 people that you're just going to go to when you need some help. It's really critical," she said. She'd also send them routine updates on where she was with her business to stay in touch.

The best investment you can make is your spirit

While Fleiss said it's important to invest in your product, team, and facilities, there's one investment that she's found to be priceless to any business's success: faith in the mission.

"The most important investment is making sure that you love and believe in the business you're working on, because you're investing your own time," Fleiss said. "Whatever you do in life, that will always be the biggest gift, the most scarce commodity, and the resource that you need to optimize around."

Stay customer-obsessed

Fleiss has been both an investor and a founder seeking funding. Through it all, she's found that her focus on customers has brought the most success. When choosing which companies to invest in, she looks for this customer-obsession mindset above all else.

"You want to know that this person is not only obsessed with the customer and the problem they're solving initially but that they're going to keep being focused on going back to the 'wow' and understanding again and again and how to best serve the customer," she said.