Ahh, you can't have it both ways. Does a simple trend line, e.g. 12 month moving average, suffice to tell you to get in or out of a stock ("the bear market that ended in 2009 was easily identified by the breaking of trend lines")? Or does it have to be the right trend line? Moreover, how do I know what trend line to choose? I can choose great trend lines with the benefit of hindsight.

Also, forgetusername's point on false positives remains critical. Breaking trend lines is only a useful if it's right most of the time.

My advice to anyone who wants to be an enterprising investor (i.e. put more time into investing and try to achieve higher returns) is to read Ben Graham's The Intelligent Investor. Instead of trading based on the emotions in the market, invest by choosing great companies that are bargains.