@YJ - sell blood and semen. They are renewable resources.
The special added benefit of selling blood is that, more than likely, you will be using your new-found wealth copious qualities of alcohol and illicit drugs.
As was much studied in my college days, this is where the famous "Buzz-to-Dollar Ratio" becomes a particularly relevant factor, and it's a wonderful double whammy.
First, obviously as you give more blood, you will be increasing your dollars, thus increasing your ability to acquire whatever your favored source of buzz is.
But...and this is the beautiful part...the more blood you give, the more susceptible you will become to achieving the desired buzz.
Thus...the more blood you give, the higher your BTD Ratio becomes, meaning it requires less dollars to achieve the buzz.
Thus, giving blood is your surest way to wealth and happiness.
As a side note, there is often a concern from the uneducated who will say, "Yeah, but Mass'ah Krellin, isn't giving blood work?" Tsk tsk tsk...Giving blood is not different from your daily activity: sitting on the couch.
A good blood donation clinic will often comfortable chairs, some sort of viewing media, and if your lucky, a tight-lloking college girl/med-student to draw your blood. Nothing like eye-candy to pas the time, right? And if that isn't to your liking, bring your favorite MP3 player and just chill.
Chances are, you're already slightly buzzed coming to the clinic, so I recommend the tunes - helps set the mood for whatever BTD activities you will be partaking in with your crisp new dollars in hand.
OH...and I almost forgot - FREE COOKIE when you done.
Damn...what a better way to become wealthy and content!