Dark nail polish, smoking or exposure to harsh cleansers can stain your nails.
You can prevent yellowing by applying a base coat before any nail polish. To remove discoloration, try soaking nails in a paste made from hydrogen peroxide and baking soda. Whitening tooth paste can also help remove light stains. You may also want to invest in a 4-way buffing block which will help remove stains and restore a healthy, natural shine.