Observe, ask questions, maybe learn a technique or two that dental auxiliaries are allowed to perform without formal training, like pouring up stone casts from impressions.
Want to really impress your dentist? Ask him/her about the principles behind cutting the teeth to prep them for fillings. They don't simply drill a hole and fill it-- Cavity design is CRUCIAL in helping a filling stay in the tooth and is a lot more complicated than most people realize. And if you want to be a teacher's pet, do some research on a gentleman named Greene Vardiman Black beforehand.