When a person begins to workout there may be a number of reasons why the person eventually quits.
It could be due to not making enough time to workout.
It could be that the person needs an exercise partner to help him or her keep up with the consistantcy needed to eventually make something a habit.
Here's a video from someone that is dealing with motivation and consistancy.
For me I stay motivated by visiting exercise blogs and keeping track of my progress with an excel spreadsheet. The bigger guys at the gym also motivate me to keep up with my workouts and to workout harder.
One of the hardest parts to working out for me is to vary my workout to keep losing fat. For a while I was running at a constant speed on the treadmill but could not get past the 192-193 lbs area. I went to Vitamin World and they suggested doing interval training and to vary speeds when I run as well as doing sprinting. Let's hope I can continue to lose weight and body fat.