For sure, trading plan is extremely important part of trading process. Each trader should have a detailed trading plan created before entering the position. A properly developed trading plan would guide a trader through all the stages of trading process from choosing the setup and entering the position to working with position and closing it with stop loss or take profit. To my mind, most of the newbie traders fail just due to the lack of planning. Even discretional traders should have trading plan since it helps them to avoid making decisions while being in the position - such decisions could be influenced by bias caused by current position (trader would unintetionally search for the confirmation of his initial assumption to feel comfortable about his position).
That is why trading with a plan is the first way to reduce the impact of emotions. If the trader acts strictly in accordance with his plan, his emotions would not influence substantially the trading process in general and the results in particular. At the same time, trader should be disciplined enough to be able to follow the rules otherwise he would mmore likely to violate the rules and make emotional trades (which are mostly losing ones). So the trader interested in improvement of his trading performance should focus on discipline.