Train for an event. Training for a marathon, bike race or other fitness competition will give you a reason to stay on task week after week. This is a great technique for people who are motivated by deadlines.
Set goals for your appearance. Train to lose a certain amount of weight, get six-pack abs or look good in a bathing suit by the summer. In the short term, many people find the prospect of a leaner, more attractive body a great fitness motivator. Let your vanity work for you.
Is it Fun?
Do something you really enjoy. This may sound obvious, but it is a fitness strategy that many people miss out on. If you find yourself hopelessly bored running on a treadmill and lifting weights, take a dance class, bike around your neighborhood or jog outdoors. If you aren't enjoying your workout, you probably aren't going to stick with it.
Let competition motivate you. If you're the highly driven, competitive type, work out with a friend who is another "Type A" personality. Through friendly competition, each of you can give the other motivation to work that much harder. As long as no one takes it to a personal level, there's no reason why you can't be friends and fitness rivals. Or use your fitness to further competition in other realms. For example, work to get stronger as a way to improve your golf swing.
Find motivation in camaraderie. If you join a team or a dance group or sign up for a class, you'll feel part of something. Many people work harder on their fitness goals if they feel a sense of obligation to a group. For people who aren't competitive by nature, having a workout partner inspires feelings of teamwork instead of competition. Both buddies can work together for mutual benefit.
A Little Bribing
Bribe yourself. In the long run, bribery is probably not the best motivator, but it can get you started. Treat yourself to a nice meal or a night out every time you successfully complete your fitness goals for the week, for example. If done in a more subtle way, bribery can actually be a powerful psychological motivator. For example, if you do a hobby you really enjoy after you have worked out, your mind will associate that hobby with exercise. Therefore, just exercising will make you feel some of the pleasure you get from the hobby.