With regards to the workouts, I try to never have a client reach a plateau. There are simply so many variables and variations of such that reaching and being stuck at a plateau seems impossible.
Now, a weight loss plateau is a different story. I have a client that has lost 25-30 pounds while working with me (over a year or so). Sure, she gets frustrated, but she knows how far she has come. We continue to focus on the positive aspects of her health and body, while encouraging change in the areas she wants to focus on. The best we can do is be diligent with diet (as she has), and keep changing things up exercise-wise so that her mind and muscles never get bored or complacent.