This tip has been around for years, but it's still as good for making extra money today as it always has been. Some people give blood in order to help people who need it in surgery, and for medical treatments. If you're one of them, well done. That's very commendable. But you could take it one step further and sell your blood plasma.
The plasma in our blood is used to treat a number of diseases, both large and small, and it's valuable. There are plasma centers all over the country (and all over the world), and they'll set their own rates for donor payments, but it's not uncommon for a donor to be paid up to $400 for each donation. It's not the most pleasant experience to go through, but you can make quite a lot of money doing it.