Jon has given great advice about rentals being a cash on cash game. To expand my cash flow on limited captial, I have focused hard on the cash on cash number. As Jon said, I have fund that you can do better at this by managing yourself and doing so in the lower working class neighborhoods. This means I don't invest in the neighborhood that i want to live in or to people that I typically run in a social circle, but its still not a scary place to be at night. The hassle factor is a little higher, its not as easy to sell these properties to an owner-buyer on exit, but the cash flow as a function of purchase price is much greater. As a result, I can get greater than 15% cash on cash return.
The increased return has its draw backs (property management, increased hassle, and lower liquidity, ect).
Another thing you can do to increase cash on cash is to finance with a mortgage. This route you can lock in a lower interest rate for a fixed period of time. There are some downsides to going mortgage as well... higher closing fees, and no LLCs.