There are a few natural ways to prevent yellowing teeth. First, try brushing your teeth with baking soda at least once a week. This will help to remove any surface stains that may be causing your teeth to turn yellow. You can also try using whitening toothpaste, which will help to brighten your smile and keep your teeth looking their best. Finally, you can try eating more crunchy fruits and vegetables, which will help to scrub away any plaque or bacteria that may be causing your teeth to yellow.