Michael Marcus mentioned some important rules in Market Wizards: Interviews with Top Traders.

Perhaps the most important rule is to hold on to your winners and cut your losers. Both are equally important. If you don’t stay with your winners, you are not going to be able to pay the losers.
You also have to follow your own light. Because I have so many friends who are talented traders, I often have to remind myself that if I try to trade their way, or on their ideas, I am going to lose. Every trader has strengths and weakness. Some are good holders of winners, but may hold their losers a little too long. Others may cut their winners a little short, but are quick to take their losses. As long as you stick to your own style, you get the good and bad in your own approach. When you try to incorporate someine else’s style, you often wind up with the worst of both styles. I’ve done that a lot.