Bluing. You can find it with laundry products at most grocery or department stores. Mix a little bit - like a couple of capfuls (from the bottle it came in) - in a gallon of water and scrub the stains with that after you've used your regular soap, then rinse it out really well. It works great, but if you use too much or leave it on too long you'll end up with blue stains instead of yellow ones.
You can also look wherever livestock or horse supplies are sold - there are a number of whitening/stain removing shampoos available. Be sure to follow the directions on the label, as some have bluing or something similar in them and are capable of staining if not used correctly as well.
