Advice from the WSJ health column:
"When you eat or drink something acidic, the pH in your mouth goes down and can take some time to go back to normal." The ideal pH of a mouth is about 7, while a soda—even a diet one—can be as low as 2.5 or "about the same as household vinegar," says Dr. Cole. Acid demineralizes and weakens the tooth surface, making it more prone to decay.
Instead of brushing (which, more than once or twice a day is hard on your teeth), they recommend rinsing your mouth with water to neutralize the pH back to levels that aren’t so helpful to cavity-causing bacteria.