Most public water supplies now contain a small amount of fluoride which helps to strengthen your teeth.
Decreasing the amount of consumed sugar will prevent the bad bacteria from producing acid and destroying enamel.
Decreasing the frequency of sugar has a larger effect in decreasing tooth decay. Whenever your mouth bacteria are exposed to sugar, they produce a bit of acid, if the bacteria have constant sugar exposure, your teeth will erode.
Chewing gum stimulates your saliva which will help clear away some lingering sugars and food particles in your mouth. Like brushing, it can also physically remove any loose bacteria.
Dairy products are great for restoring the acidity and reduces the risk of tooth decay. Cheese is particularly good as it contains a protein that helps with retaining minerals that help repair and strengthen enamel.