After careful investing, I lost 1/3 of my savings and IRA's in two major stock market crashes, so I learned this simple system and never lost money again. I keep all my money out of the market until there is a major crash (below 8000). Then I buy a no-load Stock Index Fund. I may wait years, but when the Dow reaches 10,500, I sell immediately and stay out of the market until the next major crash, then do it again. You might feel comfortable with a different magic sell number--10000 maybe?
Even though this system has never lost, it is because I never got greedy. I do not try to guess when the market will hit its lowest, or its highest, I just go by my moderate magic numbers. I also never invest more than half my savings. Whoever said "buy low, sell high" meant it.