As with any opening, it is important to immediately catch your audience's attention. How you choose to open will depend on two factors: your target audience members - are you addressing a group of young children or a dentist's convention; the overall tone of the speech - are you going for a serious informational speech or one that is more relaxed and humorous.
You could start it off with a question to get people thinking - "Up and down? Back and forth? Circular motion? Which is really the most effective way to brush your teeth?" Maybe use some audience participation by having a show of hands with a younger audience. Younger children will be more inclined to listen to see if their answer was the correct one.
Perhaps start with some dental humor; even a corny joke followed by a "but seriously" will get your audience listening. Depending on the age of your audience will determine what kind of joke you could tell.
If it is a more serious presentation, then I would suggest finding a shocking/interesting statistic or quote that would be appropriate to the audience.
Hope this helps. Good luck with your speech.
Some interesting ways to introduce the topic seem to pop into my mind. The first would be that brushing one's teeth represents the natural start to any day and would be an appropriate start to the speech. Another way to introduce the topic would be to place the listener in the middle of the action, and discuss how the day unfolds with the brushing of one's teeth. This could take a variety of forms, with discussion of how one awakens in the morning, the act of brushing one's teeth, and the thoughts and actions that might go into that. Perhaps, a very post modern way to open it would be suggest that the introduction to the day starts with brushing the teeth successfully and an introduction to the speech should also open successfully. This would have to be an artful way of presenting the topic and might be difficult to pull off, but if the audience is savvy enough to grasp it, there might be some success evident.
The introduction of any speech needs to serve two purposes. First is to inform what the topic of the speech is, and the second is to get the audience interested in listening to this topic. I believe both these purpose can be served by staring with pointing to the importance of oral hygiene, and the importance of brushing teeth regularly and properly for maintaining good oral hygiene.
This objective can be achieved in many different way such as using humour, starting wit a question, narrating a short incident, or just plainly stating the importance of oral hygiene. But do not loose track of the two central objectives of introduction of a speech.