What is the best way to use the Singing Success CD's? Should I practice the exercises on just one CD at a time (say for a week) and then move on to the next CD? I'm currently practicing the exercises from two CD's at a time and then adding one exercise from the next CD each week, while dropping one exercise from the earlier CD's. But this involves about an hour of practice each day and I'm afraid that may be too much. Also, as I move forward, do the exercises on each CD take into account warm up? Thanks!