Don't wait until you're close to your prescribed sleep time to start your pre-sleep teeth ritual. Get it done as soon as you can. (Put a music player in your bathroom to make brushing and flossing something to look forward to.) Doing your teeth right before you go to bed breaks any momentum you have toward sleep. When you're tired, ready for bed, when your eyes are feeling heavy and your mind is filled with images of nighttime, brushing and flossing can shake you from your pre-sleep state. Bushing and flossing can make you feel more alert. When you're feeling ready for bed, you don't want to do anything that makes you more awake.
Getting your teeth done as early in the evening as you can will help keep the path toward sleep a smooth one.