Make an appointment with yourself. Write down upcoming workouts in your planner as if they were business appointment. Don't be ruled by the scale. Since muscle weighs more than fat, as you get fit, the numbers on the scale may lead you to think you are not truly losing. In addition to the scale, track your progress with a measuring tape and by the fit of your clothes.
Rewards are great incentives.
Tell friends and family about your get-fit goals. Support and encouragement from loved ones is a big motivator. Think of yourself as healthy and fit. When your willpower starts to wan, bring up those images and don't let go of them. Wear a heart rate monitor when you exercise.