Tactical Mastery Tip #1 - Know the goal. The mission should have a set goal.
Gygax states that when the goal is successfully achieved, the mission is complete and the adventure should conclude at that point. That's pretty direct.
He acknowledges they can overlap, but to differentiate mission from goal, he says that the mission is a description of what must be done to achieve success. The goal is an enumeration of the conditions that will prevail when the mission is complete.
For an example, he talks about the mission being to catch a criminal. The goal of the mission is accomplished when the criminal is behind bars.
I know I've played in groups where our goal for a session or the task in front of us was never defined. We basically moved ahead and looked for monsters and treasure. That's certainly not tactics. Not much of a strategy either, for that matter.
Knowing the mission and defining the goal leads into his next tactic, which relates to objectives.