The following is a big one.
Meet my best friend:
Please note that this is Oxi Clean in its powdered form.
In my house, we are a sweaty people. Those white undershirts? With the yellow stains in the underarm and collar areas? A good long soak in an Oxi Clean solution, then a good wash in the machine takes right care of that problem. Blood stains (and any other bodily function stains)? A good soak in an Oxi Clean bath will take care of those too. Even stains that have been on clothing for awhile and have been dried in the dryer first. Oxi Clean is very good on organic stains, i.e. coming from humans (blood, sweat, bathroom stuff) and food (although Dawn dish soap is my favorite for food stains since they so often come with grease).
Here is what I do: I buy this large box at my warehouse store. I use a 5 gallon bucket and fill it with about 1 cup of oxi clean powder and hot water. I can soak 7-10 undershirts at once. Or a sheet. I give it a good long time because I don't want to have to do it again (usually overnight or most of the day). I could probably use less powder, but again, I don't want to have to soak it again, so I make sure there is a good amount in the water. When it is done soaking, I squeeze the garments out, and throw them in the machine and wash as usual using warm water. The stains are gone (for awhile anyway), and the whites are whiter too.