Sounds like the easiest and most simple principle to understand.  That is, until you are looking to execute it.  Imagine this, you see a stock you like that is getting some bad PR and subsequently, the stock value is beginning to drop.  A stock that was once $50 is now $25.  The question to ask yourself, do I buy now, or wait for the price to keep dropping??  Let's just assume you buy it at $25 and it continues to sink on more bad news, which might just turn out to be rumors.  Now the $25 stock is trading at $20.  Should you have waited to buy?  Do you sell now and look to buy more when it drops even lower?  Do you like it more at $20 than $25 and decide to purchase more?  Buy low, sell HIGH right.  When do you buy?  When do you sell?

When I find the perfect answer, you'll be able to buy it in my book, for $19.95!