It's much easier to learn what you should do in trading than to do it. Good systems tend to violate normal human tendencies. - William Eckhardt

Paul Tudor Jones:

Don't focus on making money; focus on protecting what you have.

Don't ever average losers. Decrease your trading volume when you are trading poorly; increase your volume when you are trading well. Never trade in situations where you don't have control. For example, I don't risk significant amounts of money in front of key reports, since that is gambling, not trading.

If you have a losing position that is making you uncomfortable, the solution is very simple: Get out, because you can always get back in. There is nothing better than a fresh start.

The most important rule of trading is to play great defense, not great offense.

Don't be a hero. Don't have an ego. Always question yourself and your ability. Don't ever feel that you are very good. The second you do so, you are dead.