Variety is the key!
Changing your workout programme approx. every 6 weeks is the key to not only alleviating boredom but it will also keep your body guessing. so there is no room for the dreaded plateau!
Depending on what you are training for, make sure your workouts are balanced between all muscle groups and you are incorporating some intensity work during each week to keep the metabolism fired up!
Once your body has got used to a movement, for example steady state running, it will become inefficient within about 4 weeks as the body adapts quickly to the task in hand. So at this point you may want to add in an interval session once per week replacing one longer run. This will give you a greater calorie burn out and it will change your metabolic state meaning the body has to work harder to recover. End result = you getting fitter and stronger as performing the intervals will be great for building muscle too.
If you are weight training and have started with a base programme with the intent of building muscular endurance, you may be doing around 15 repetitions per exercise. After 4-6 weeks make sure you are adding more weight and reducing your reps down. In addition to this you can vary the exercises you are doing too. For example if you started with a reverse lunge you could progress to walking lunges holding weights.
There is no right or wrong answer here, but definitely adding variety to workouts will be a great starting place to keep the body guessing!