Hi Jessica!
First you should evaluate your situation at the 12 week mark. If you are still seeing progress then you can stick with the PS Series. However, if you feel that your hitting a plateau and your muscles need to be shocked with something else for a while, then yes, by all means, shake it up.
When you are ready to shake things up, I think MIS and Bodymax are great tapes to alternate. Its a complimentary combination of cardiovascular endurance and muscular endurance. I would suggest that you do these workouts every other day. EX. Monday MIS, Tuesday off, Wed Bodymsx, Thurs off, Fri MIS, Sat off, Sun Bodymax, Mon off. After two weeks of that pattern, cut back to one MIS and one Bodymax per week, while crosstraining with other activities the rest of the week. Have Fun!
