http://www.youtube.com/watch?v=CduA0TULnow
This is a link for the video of Brittany Spears "OOPS Did It Again". The reason she is singing incorrectly in this video is because she is straining the vocal cords. When she raises her head or neck or lowers it to low she is causing tension on the neck thus cutting air way and you can hear it in certain areas in this song.
http://www.youtube.com/watch?v=KtNYA4pAGjI
Martina McBride, "Concrete Angle" is a video of correct vocal techniques. Martina's posture is good and sh seems to keep her air way clear and the sounds come out crisp and clear.