Market timing is very difficult (really it's luck), but that doesn't mean just throw all your money in at one time. I saw the end of the good days when, in 2007, the DOW broke 12,900 - and I was out of there. I put everything in cash and didn't get back in until late November 2008.
When I left the market, I watched it rise another 5%. And when I got back in, I watched it hit new lows in March of this year. I don't believe anyone can time the market, but I do believe you can know when something is overblown.
In 1999, if you believed the NASDAQ was overvalued, you would have watched it rise for over another year. You would have missed out on a 100% move! Or would you have? Holding would have reduced your NASDAQ holdings by 50% (that's NEGATIVE 50%) by 2002.
So what do you do today - right now on October 6, 2002. The fact is, the market is extremely over bought and shouldn't be trading any higher than 950 on the S&P. Another fact is that cheap money, courtesy of the FED, is making the stock markets into the next bubble. Bubbles can last a LONG time.
You're not wrong if you leave the market today - though I would rec'd getting into an inflation hedge. Inflation alone can boost this market by 100% in the next few years - with no real value to your portfolio, but a definite loss to those holding cash. Yes, the FED (aka: 4th arm of government) is making smart investing very hard, but if you believe the market is overvalued (and IT IS), then get out of it now and invest in gold, food and energy. Those three things have been around for a while and I suspect they'll be around for a while longer
Good investing to all!