Investing is often done best when it's kept simple. For the average person out there looking to boost their portfolio returns, this could mean putting on your customer hat and finding businesses that you already deal with. Not only does this allow you to lean on your own first-hand experience, but it limits the universe to companies that you probably understand very well.

This will undoubtedly lead you to the retail sector, which is extremely competitive. But it has produced some outstanding winners historically. In fact, there are two dominant industry heavyweights that have turned their longtime shareholders into millionaires.

The first millionaire-maker retail stock is Costco Wholesale (NASDAQ: COST). The company has produced a total return of 18,030% in the past 30 years (as of June 25). Had you made an initial investment of $5,600 in the summer of 1994, you would be sitting on a balance of $1 million today.

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Source Fool.com