Eric Franklin, the Franklin Method’s progenitor, recently released a couple of short videos addressing the issue of core strength and breathing which I think are very useful for singers.
In this first video, he shows the relationship between the diaphragm and the abdominal wall muscles, and leads you through some breathing and movement exercises pertaining to the same.
In the second video, he shows the movement relationship between the diaphragm and the pelvic floor muscles, and again leads you through some basic movement and breathing exercises.
If you enjoyed these videos, I suggest heading over to the Franklin Method website and subscribing to their video library.