Statistically speaking, over 50% of those who begin a workout routine end up stopping it within a year.
When it comes to exercise, the main culprit -- along with simply not doing it, is not doing it enough. Motivation and a pesky lack thereof is what keeps the majority of people exercising, consistently, even when they've already begun a routine. Some people were simply bred to workout, constantly; for the rest of us, we're going to have to find ways to keep motivated.
The key is to find an activity that meets your individual needs. In that last sentence you will find the first step in staying motivated with your workout. You, that's right, the person in the mirror, needs to decide, without help from the outside world what activity is best for you. By making the decision yourself, you will not feel like you're being forced into doing the exercise and therefore, be more likely to stick with it. Think about it, most people are ready to defend their own ideas as opposed to another person's, aren't they! Next, you'll need to decide what you're good at. Everyone has something they are more inclined to do than other things. Like shooting hoops more than running? Well then, that's what you need to do. If it means buying a hoop for your driveway or getting to the park for a little pick-up ball, there's your workout. Now, just implement it into a routine and your halfway home. Finally, by utilizing what you know you're good at, find other, likeminded individuals and create a group around the activity you're trying to become involved in. Having a community will create in you the desire to do well around them and an eagerness to make sure they are doing as well as you. This support group will be a great fundamental way to ensure you stick with a program.