These real estate investors built cash-flowing portfolios. Here are the rules they swear by.

Jeff White and Suleyka Bolaños say renting individual rooms has helped them maximize rental income and cash flow. Jeff White and Suleyka Bolaños Investors use rules like the 1% rule to screen for cash-flowing rentals. Some boost cash flow by adding units: "The more doors on the land, the more money it produces." Others see mid-term rentals as the 'sweet spot' between long- and short-term leases. Producing cash flow from rental property sounds simple: collect more in rent than you spend on the property. In practice, getting the numbers to work can be much harder. Investors have to find a property at the right price, finance it, account for ongoing expenses, and keep it occupied. Business Insider has spoken with real estate investors across the country about the rules they use to find — and maintain — cash-flowing rentals. Here are four that they swear by.