Use rental cash flow to pay down 6.375% mortgage?

[Craig Daniels]

Hi - This was partially answered on one of the BP Podcasts recently, but I was curious to get a few opinions.  I just bought a rental for 30% down and have a 6.375% 155k mortgage on the place.  The rental will cash flow between 200-300/month.  Looking at pure spreadsheet math and common convention; if you pay down your mortgage you are effectively making that interest rate on your money.  If I were to put 2400 per year back towards the mortgage, the 6.375% equates to only (maybe) 150 per year that I am "making", but if you do that for a few years up front you can shave off years on the back end of the mortgage.  

I get the 2 schools of thought on keeping cash flow to stay liquid vs. investing your money where it will make the highest return, but my W2 job salary means I don't "need" the 2400/year of cash flow at the moment, really all I am looking to do is maximize this rental return over a 10-15 year return window.  

Anyone in a similar situation and thinking along these lines?  Curious to hear the forum's thoughts!