Examples of parameters which can be altered include repetitions, loads, sets, exercises, exercise order, and rest periods. In my experience, it's best to change only one variable at a time in order to accentuate muscle fatigue. For instance, keep all the aforementioned variables constant but increase the loading on those exercises in which you can perform more reps than the planned rep range. In this way, you can easily track the increase in weight needed in each exercise to adequately fatigue your muscles. Inevitably your strength will reach a threshold in each and every exercise. Only when your max weight has been ascertained for every exercise should you consider moving on to another workout program. If your workout program calls for hitting the same muscle group more frequently (i.e., working chest twice weekly), consider varying the order of your exercises to reduce stagnation and boredom.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."