New Market Wizards is worth a read

 Some guide lines  Two secrets of trading success:
1. Preservation of capital and Home Runs.
2. It is not easy to buy a stock after you have sold it.
3. Use left brain for fundamentals and right brain for
imagination and story behind how a stock will soar.
4. Victor Sperando's goal in Wall Street is not to get
rich but to stay in business.
5. The key to success is not knowledge alone but
emotional discipline. The best analogy to this is
dieting. You know that to slim down, you need to
exercise and avoid fats. But the knowledge of this
does not make you slim. It is the discipline to do
it that counts.
6. Have a balanced life.
7. Do not buy a stock that is going down even if the
fundamentals are good. (when broker houses are
issuing 'buy' calls)
8. Never use valuation to time the market.
9. Be able to get over a loss in twenty four hours.
Otherwise, the position is too big.
10. Emotional makeup and discipline is more important
than IQ. (i.e. IQ is still important)
11. To win, you have to be the minority contrarian.
And the most important is this:
12. Watch idly when profit is running. When losing,
run like a jackrabbit. You do go broke taking a profit.
Let profits run!

I have a question here. How does one let profits run?
How does one know if the price will not reverse and
your profit turns into a loss?