Dollar Cost Averaging
Dollar cost averaging (DCA) is the simplest and most effective approach to investing for 99% of market participants.
Investing your full position all at once can be incredibly dangerous and overexpose you to the dangers of natural market volatility. Nobody knows if the market is going to move up, down, or sideways, so a much safer approach is to invest a small amount of money every week/month or whatever duration you find appropriate.
By investing a small percentage of your total position at a time, you will all but guarantee a favorable average price and protect yourself from excessive drawdowns.
If, for example, you were planning on investing $10,000 in AAPL, buying all those shares at once could easily lead to several thousand dollars of drawdown in the short to medium term. AAPL could experience a 20% decline over the next few months and you would be left kicking yourself for not waiting and securing a better price.
With DCA, however, you won't be committing yourself to any one price. You might buy $200 of AAPL every week for the whole year and, in doing so, be far less affected by any large price moves. Whether the stock moves up or down, you will be buying a small portion of your position periodically and essentially get the average price of AAPL over that entire period you are DCA.
Generally speaking, timing markets is a fool's game that usually leads to pain. DCA saves you from much of that pain and takes a lot of the emotion out of investing. It's a simple plan anyone can follow and has a long history of outperforming lump-sum investing.
Even the most experienced traders use DCA as a strategy for their long-term investments because it really does work. Normally, people hate to see their investments go down, but for the DCAer, that just means you get to lower your average price on your next purchase.
Even if you only break your investment up into 10 purchases, you will almost certainly find that you get a better average price all the while avoiding excessive drawdowns that hamper your portfolio's performance.
Keep things simple and protect your money by dollar cost averaging into all your investments.