Starting off on a healthy lifestyle is easy enough. Sticking with it isn't. Many people set goals for themselves, yet never reach them (think of all the New Year Resolutioners at the gym who disappear after a month or so). How you can you be sure that you won't become one of them?
Hold yourself accountable. Don't just set a goal, set up checkpoints along the way to track your progress.
No matter what your goal is, there is some measurable variable that you can track. If you're looking to lose weight, it's the number on the scale. If you want to lose bodyfat, it's your bodyfat percentage. If you're trying to increase a lift, it's the amount of weight you load on the bar.
The key is to set the checkpoint at the right time. Daily checkpoints aren't the best way to reach your goal. Not a lot changes on a daily basis, so it can be de-motivating to track daily progress and not see results from day to day. However, if you set your checkpoints for, say, every 4 weeks, you will see results (if you're really sticking to your plan). Results translates into motivation.
If you keep a physical record of your progress, this will help keep you accountable for following through on your plan. Personally, I like to keep workout journals online where I not only track my workouts, but I also track my goals. I find that posting a goal, my plan to achieve it, and my periodic progress keeps me on track. Not only am I holding myself accountable by putting in black and white where I stand, I'm also putting it out there for others to hold me accountable.
How you track your progress is up to you. Just be sure to set up a system that will keep you focused and motivated.