The Gambler/Systematic Trading, from Phil McDonnell
At one time the Chair recommended a 3 strikes and out rule. If your system has lost three times in a row it may be tired and needs a rest. Another way to monitor that is more sophisticated is to use the techniques of industrial quality control. Essentially you monitor your average expected gain via something like a moving average and then take the standard deviation around that. If your average ever wanders outside the 2 sigma band above or below then your process has failed and needs to be reviewed. The idea of Shewhart diagrams is similar to this.