Since you brush your teeth (ideally) at least two times a day, it's an opportunity to be mindful for at least a minute each time. Think about each act you are doing deliberately as you do it, the New York Times recommends.
Focus on the toothpaste as you squeeze it out onto your toothbrush. As you brush, breathe in through your nose slowly and breathe out. What does the toothbrush feel like? How do the bristles feel against your teeth and gums? When you're done, pay attention to how your clean teeth feel.