Keep your risk low enough so that if you lose ten times in a row you will not be psychologically or financially wiped out. (And if you do lose a few times in a row, make the risk lower and lower till the losing trend breaks.)
This is an excellent explanation of why one should try to exit losing position as early as possible. "It costs enormous mental and emotional capital and opportunity cost to babysit losing positions that wears us down and we often make poor decisions because we so badly want the position to work." This alone is the reason enough to exit all losing position as fast as possible and fish elsewhere. Clinging on a losing position for more than necessary always brings negative emotions, makes one less firm and hence make more bad decision. Its better to exit and wait on the sideline than get hammered.
“I’ve learned many things from [George Soros] but perhaps the most significant is that it’s not whether you’re right or wrong that’s important, but how much money you make when you’re right and how much you lose when you’re wrong.” - Stan Druckenmiller