It is probably significant that our top two favorite trading books are written by a psychologist and a psychiatrist! According to Mark Douglas, author of The Disciplined Traderand Trading in the Zone (see books), trading success is 80% psychological and 20% methodological (your trading edge). Others might dispute those percentages, but everyone would acknowledge the huge part our emotions and our mindset play in trading. If asked to picture a successful trader, you may call to mind someone with a phone on each ear, manically screaming, "Sell, sell!" However, a more suitable image to aspire to would be Star Trek's Spock - his Vulcan side knows no emotion, and that should help to keep his human side in check.
It is extremely important that you are mentally prepared for the trading day. Don't just arrive at your seat one minute before the market opens and expect to be successful. It takes mental preparation, a run-through in your mind of what you see unfolding according to your Daily Game Plan. If you can find the time, just say 10 minutes, for a mind-calming meditation so much the better. If you are feeling at all stressed due to say a traffic incident or an argument with your spouse, or if you are feeling 'under the weather', don't trade until you regain your equilibrium.
As you go through the day remember - be rigid in your rules and flexible in your expectations - not the other way round!