A strong, clear action plan is critical for goal success.
Your goal is the what: what you want to achieve. But the action plan is the how: how you are going to achieve it.
A goal without a good action plan is merely a wish. And it's guaranteed to fail.
But what makes a good action plan?
Planned ahead. The more you think ahead, the less likely something unexpected will obstruct your goal. Are you going on a holiday in the middle of a diet goal? Factor it into the plan.
Self-aware. Setting the goal is the easy part. But once you're in the middle of it, you'll start finding excuses not to do the work. So preemptively think of how you'll weasel out of what you've committed to. Will you say "I'm too tired because of work" so you can avoid the gym? Factor it in. Go to the gym before work, or drive directly there after work.
Thick. A thin plan, with one or two brief action items, isn't enough. The action items should be clear and well thought out. They should be many.
"Will not smoke" is a useless action plan. A good plan would be: (1) won't buy cigarettes, (2) won't hang out with smokers, (3) will buy & take nicorette, (4) will go for a jog whenever I feel cravings.
"Will go to gym" isn't good enough. A good plan is: will go to the gym on Monday, Wednesday, and Saturday, and will complete a minimum of 12 sets of mixed exercises or 1 hour of aerobic.
Robust. The goal shouldn't be fragile. Maybe you have a late meeting on Wednesday, and can't get to the gym. The plan should factor that in: go on the next available day.
Ramped up. Jumping into a goal all gung ho is great, but not if it scuttles the goal. Hitting the gym hard after being a couch potato for 3 months will result in epic DOMS that will cripple you and prevent you sticking to your action plan in the following week. So ramp it up. Steadily add more action each week, until you're at the full action plan.
On the other hand, some goals like "quit smoking" respond better to shock therapy.
So in conclusion...
A solidly thought out plan like that can seriously boost the odds of your goal succeeding, and you getting the results you want.
Treat the crafting of your action plan as part of the work.