Every time you eat or drink something sugary, bacteria on your teeth produce acid for twenty minutes or more. Saliva then slowly repairs the enamel. Decay happens when the acid attacks come too often for that repair to keep up.
Keep sweet things to mealtimes
A dessert after dinner is one acid attack. The same amount of sugar sipped through the afternoon in a coffee or juice is a dozen.
Water between meals
Water, milk, plain tea and coffee without sugar are the safe drinks between meals. Sugar-free gum after eating helps saliva do its job.