Advice On Body Shock

I have read that in order to get full results from work outs you need to keep your body in shock, keep from getting used to the workouts you do. If you have experience with this I would greatly appreciate advice. My questions are:

1. How long should you go with one workout before changing it up?
2. Are different work outs in one week a good idea?
3. What are your thoughts on DVD workouts? (I've done Jillian Michaels' 30 Day Shred, some Zumba, Kick Boxing and the like)
4. How much running is too much? Should I do it daily or slack off a bit?