Have you had your teeth cleaned (scaled and polished) by a dentist within the last 6 months? If not, that's a good place to start. No funny business, they just remove tartar and plaque. The buildup.
Besides that, you can try oil swishing (an oil such as sesame, olive, or other, swished/swirled/pulled in mouth about 10-15 minutes) once or twice daily, as well as baking soda and hydrogen peroxide brushing (at a different time of the day than the oil swishing).
I would think you can get more suggestions here. But don't rub lemons/lemon juice directly on your teeth, always dilute with water.