Make those yellow stains from dark polishes disappear in two easy steps. First, get a small bowl of water, add a splash of hydrogen peroxide and soak your nails for 10 minutes. This will mildly (and very gently) bleach away the discoloration.
For more stubborn stains, grab a nailbrush and some whitening toothpaste and rub briskly across your nails for a few minutes before rinsing with water. In the future, make sure to use a base coat to protect the surface of your nails.