Give yourself days away from your exercise program. Again, depending upon goals and level of fitness, rest at least 2 to 4 days each week. This is a powerful reward to a job well done. Schedule a massage. Walk 18 holes of gold instead of doing cardio. Take in a show or go out to a fine restaurant; all ways to pay your body back with play for all the work it's done. Be sure to connect the two events in your mind. Make a conscious effort to associate the fun with payback for the work you've accomplished.
Every once in a while give in to your urges. Occasionally, and only because you've been so on the money with your program, give yourself a bonus. Remember, this has to be a spontaneous event or it falls into the category of a scheduled reward. Every now and then, after a particularly productive period, you can do something really cool for yourself. Just don't overdue it, as this will not only sabotage your program, but water down the impact of this highly effective unscheduled reward. The key here is to not only associate the play (reward) with the work (workout), but to learn moderation.
And please, let us not forget about the built-in rewards. It may take few weeks or months to actually happen, but if you use the methods I've described above to stay the course, you'll get to keep a trim, strong and healthy body for the rest of your life. So stay positive!