"Stick to the plan and let the results take care of themselves" was the instructor's advice in the Investools course that I signed Jenna up for. Taken from the quote: "I focus my total attention on trading well and let the results take care of themselves" by Tom Basso in his book "New Market Wizards".
The mantra I remember at Accenture during my days of implementing manufacturing software (specifically the MRP module - Materials Requirements Planning) was the saying - "Plan the work and Work the Plan (see blog Sept. 30, 2010).
The power of this advice is that the plan provides an objective way of stepping back from the pressure of deadlines and emotional stress about the results. In trading, the emotions of volatility in stock prices can tempt a trader to react and ignore their rules (e.g. the plan). That creates unfavorable results from the logic and strategy developed during the planning stage of investing.
Another way of justifying working the plan is that it preserves the investment of time you spent developing the plan in the first place. If you are questioning results - look at the plan and spend the time thinking about why you change the plan first before taking action.
A constantly changing plan will drive constantly changing results.