Another thing to think about is your saliva content. My best friend has a terrible time with cavities and she brushes her teeth all the time (several times a day in fact). She was told by her dentist that she has especially acidic saliva and some people can have that cause tooth decay quicker. Apparently it is a viscosity issue too. If you are dehydrated the saliva is thicker and sticks on the teeth and causes damage even more. I guess make sure you are always staying hydrated too.