I switch up my workout routine EVERY time I work out. I swim 3 days a week, but each session is a different amount of laps and different ways of swimming (legs/arms only, breast stroke, back stroke, etc.) I also go to the gym 3 days a week. I do different types of cardio each time (20 minutes at the beginning, weights, 10 minutes at the end), and also Monday and Friday are my "Leg" days and Wednesday is my "Arms" day.
I haven't gotten stuck on a plateau yet, and it makes the workouts fun!!!!