1. Take it Easy. The reason why a lot of people quit exercising is because they try for too much too soon. You know your body the best, don't push it too hard in the first few weeks. If you exercise too hard and don't give your body the proper rest it needs, your performance will start to suffer and eventually you'll feel like you failed your resolution.
2. Warm Up and Cool Down. Giving yourself a good warm up and cool down will prevent your body from becoming very sore the day after your workout. Sore muscles do a good job at making people quit so avoiding it is key during the first few weeks. Warming up also does a good job of preventing injuries as well as improving performance.
3. Exercise Without a Gym. Exercising doesn't necessarily mean joining a gym. You should find an activity you enjoy and stick with it. It can be a sport, karate or running. The more you enjoy an activity, the more of a chance you'll stick with it in the long run.
4. Before and After Pictures. Waiting to see results in the mirror won't help you very much. We see ourselves everyday so any changes that happens will be hard to notice. Other people will notice and measurements such as weight, circumferences and body fat will show improvements. If you want to track the way you look, try before an after pictures so you can see your progress and have something to compare it to.
5. Workout Buddy. A good way to stick to your routine is to find a friend and exercise with them. Its a lot easier to tell yourself you aren't working out than it is to let someone else down. Different approaches and points of view can also be helpful. Having a workout buddy will also make exercising a bit more fun.