No offense, but the average person cannot do this very well.  From Dalbar via World Beta.

Over the past 20 years investors in stock mutual funds have underperformed the S&P500 by 6.5% a year.  (8.35% vs. 1.37%.)  That return doesn’t even keep up with inflation.

They did even worse in bonds, underperforming the Barclay’s Aggregate by 6.7% a year.  (7.43% vs. 0.77%.)

This is because people want to do what makes them feel good, and what makes them feel good is to buy something that has gone up a lot.  People buy the hot mutual fund / stock / investment at the wrong time.  They chase the winners without understanding that there is cyclicality and reversion to the mean in investing. 

People should invest when they feel bad and stocks are getting hammered and out of favor, like buying stocks in March.  One can trade on momentum when one has a short-time horizon, but - with some exceptions - one should invest when stocks are cheap.