Mix some coconut oil into your toothpaste, or just use the coconut oil as toothpaste. Overall my dental hygiene habits are pretty horrid, but since I started doing that I've noticed a significant increase in overall mouth health (plus slightly whiter teeth, although that may or may not be related). It (almost certainly) won't fill in existing cavities, but it'll at least deter new ones from forming.
Also, Listerine Ultraclean. Use that once a day, after eating supper, for 1 minute. That'll help, too.