Keeping your customers happy is a top priority for every entrepreneur, but measuring that happiness is never easy.

At beauty subscription service Birchbox, one way co-founder Hayley Barna monitors customer satisfaction levels is through a net promoter score, which assigns a number between one and 10 to every customer based on a simple question: How likely are you to recommend Birchbox to a friend or family member?

While assigning a promotion score to every subscriber does help with measuring customer happiness, it's really only the beginning of a longer conversation about why customers are happy or unhappy--a follow up question Birchbox always asks.

"Our team looks at that very closely each month in order to pull out trends," Barna says. "[They're] also giving feedback to all our different departments, whether it's the merchandising team on the products that we're selecting or the marketing team on whether a promotion just didn't resonate."

To hear more of Barna's business tips, check out the video below.