I started investing in '96 after listening to Bob Brinker's MoneyTalk radio show for 5 years.

In late '98 I swtiched from Mutual Funds to Tech Stocks. In March 2000 I was up 170% in just 16 months. Even though Bob Brinker issued his sell signal I decided not to as I didn't want to pay taxes. I felt that if my portfolio dropped 50% it would be ok. I'd still be ahead and it would come back again. It dropped about 40% that year, came back close to it's high around September then dropped again by around 40% in November. That's when I decided to go 20% margin and buy the QQQ.

Fast forward 10 months and it's a week after 9/11. The stock market is about to open. My portfolio is down a whopping 80% from it's high. From the time I went on margin it had dropped another 50%. I was close to a margin call, and it was anyones guess what the market would do when it opened, so I called in my margin.
 
That basically wiped out 25% of my life savings and  pushed back my retirement plans by 10 years. If I had stayed the course with mutual funds and listened to Brinker's sell signal near the top I could be retired today. There's hardly a day that goes by that I don't think about that.

So the moral of the story is stay diversified and don't go on margin.