I hear some fitness organizations boast that they “never have their clients do the same workout twice,” while other reputable organizations promote using periodization training to create adaptation in the body, followed by the shock of change in the next phase. I get confused over which method is better, or if it’s a combination of the two ideas that works best.
The most effective way to lose weight (or more accurately fat) is to use a variety of methods. Strength training is important because an increase in muscle mass will produce an increase in metabolism. Ideally, it’s best to work all the muscle groups, but if your client really isn’t in to weight training, you can focus on the larger muscles of the back and legs to get the most bang for your buck.
Nutrition is obviously important as is the client’s eating habits. You’d be amazed that people eat for a variety of reasons, not simply because they’re hungry. Teaching a client to pay attention to their body and eat only when their body is telling them to eat is vital.
Aerobic exercise is an important component as well.
An often overlooked issue that can contribute to weight gain and makes losing weight more difficult is stress. If your client has chronic stress (not acute stress) that should be addressed.
Now . . . to answer your question more precisely, it is not necessary to keep every exercise new and never allow your client to do the same exercises. I don’t think either method is better though. Some clients do get bored more easily than others, so in those cases, changing things up frequently may be a good idea.