An interesting link to add to your collection about overtraining:
Overuse Injuries in Rowers and How to Avoid Them is an interactive site showing stability exercises with the exercise ball.
In my experience lots of people are familiar with this type of training, but can't recall the exact program to follow. This site is certainly thorough, and should help with that.
Too many athletes neglect core training, despite the almost universal acceptance of it as important.