If you are lazy you can just put your money into one of the 10+ Vanguard Target Retirement funds [1] which are just varying mixes of their stock/bond funds + automatic rebalancing. It goes from 90%/10% stock/bond mix (VTTSX) at the high end to 30%/70% at the low end (VTINX).
If you are investing new money every month and attempting to maintain the same allocation over time, you are in a sense automatically rebalancing. Of course, this assumes your total investment over a given time period is roughly in line with the relative delta in performance.
