Music can be a great motivator. Find an exercise partner. It's harder to skip a workout when you know someone is counting on you being there. Exercise partners can encourage you and push you to the next level through healthy competition. Keep a workout log.
Nothing is more motivating than success.
By logging your workout everyday, you can assess your accomplishments. You might also track the inches and pounds you've lost if weight loss is your workout goal. Fresh air and sunshine are good motivators. Go for a walk outside instead of on the treadmill. This winter, try snowshoeing or cross-country skiing rather than using an indoor ski machine.