If you're not self-employed and work for a company, find out if they have a retirement plan. If you're lucky, employers will sometimes match contributions you make into a retirement fund. Retirement plans also often have the benefit of being tax-deferred. The longer you get to keep your money (and make interest on it) the better. It's never too early to start planning for retirement.
Donating plasma can actually work out better than selling your blood. It takes longer to donate plasma because they will put your red blood cells back in your system. The upside is that you can donate plasma as much as twice a week and you can make anywhere from $40-$60 per donation. You can make that $100 in less than a week if you can stomach the needle.
If you have a blog or some other online property with some traffic, you can monetize it via affiliate advertising. For example, imagine that you write a blog about cars and you have thousands of visitors each month. You might review some books or products related to cars, and then link to them on Amazon so that you get a cut of the purchase price when anyone buys books through the links.
Start analyzing your decisions from the perspective of a firm. In economics, a firm's goal is simply to maximize profit. Well-run firms spend money only if they can expect to make more money from their investment, and they allocate their resources to the most profitable use. You're not a firm, of course, and you have other considerations, but if you make the majority of your time and money decisions by choosing the options that promise the highest return on investment, you'll likely earn more money, and that's good news for your shareholders (you and your family).
What does that mean for you? It means Nielsen will pay you $50 a year to keep their app on your favorite internet browsing device. The app itself collects statistics on your internet usage anonymously, so you never have to worry about any data being linked to you. And the best part is, the app takes up barely any space and doesn’t slow down your phone or tablet at all!