What you're asking is basically a mix of market timing and dollar-cost-averaging. Market timing is notoriously attempted (just buy when the market is low, and sell when high. Easy!) but usually ineffective because it's really, really hard to accurately predict the future of the market.
With dollar-cost-averaging (DCA) you're putting in a fixed amount periodically with the long term assumption that you will sometimes (by random change) buy while the market is down, therefore make a profit when the market recovers. While seemingly the most logical approach, a study was done by one of the investment firms (Fidelity I think?) that, on average, just putting all of your money in up front in a bulk sum will be more profitable (2/3rds of the time if I remember right) than DCA. The reason for this is, if you have the money available and aren't investing it you're losing out on potential gains during a bull market. Since the market goes up more often than down, statistically DCA is less efficient than investing a lump sum up front.
When you invest every month from your paycheck, you are unwittingly engaging in DCA. But, since you can't get paid up front for your year of work, there's nothing you can do to optimize that.
Edit: I invest lump sum at the begging of the year in our IRAs due to the aforementioned reason.