I was reading one of the books on Market Wizards and one veteran manager/trader mentioned he had something like 4 rules. They were along the lines of:
1. Never break your trading rules 2. ? Can't recall but I am sure had to do w/ cutting losses 3. another rule..... and 4. Know WHEN to break your trading rules
Of course he is saying use some instincts in trading just as in any other profession.