There is a trading technique called Dollar-Cost Averaging, implementing which allows you to buy shares (for example) below the average market price in the long run. The technique consists in regular purchases of a relatively small fixed dollar amount of shares. In this manner, more shares will be purchased at lower prices, and less shares at higher prices, so the average cost per share will be diminishing with time. I see no difficulties in applying this technique to trading in bitcoins (though I didn't try it out personally)...
This works assuming it will go up in the long run. To me that is more investing than trading. Trading consists of long and short positions so you are able to profit on the way down as well as up.
No one can forbid you from selling a small fraction of your bitcoins on downward corrections, and then again pursue this investment strategy after the price rebounds. Also, could you name bitcoin exchanges that allow opening short positions? I'm interested in firsthand experience...
Btc-e.com and bitfinex.com are probably most popular. Both offer short trading and also leverage if you want it. I prefer bitfinex as customer support is way better.