Sage advice from 37 Signals' Jason Fried on how to get good at making money through low-risk iteration and practice:
So here's a great way to practice making money: Buy and sell the same thing over and over on Craigslist or eBay. Seriously.
Go buy something on Craigslist or eBay. Find something that's a bit of a commodity, so you know there's always plenty of supply and demand. An iPod is a good test. Buy it, and then immediately resell it. Then buy it again. Each time, try selling it for more than you paid for it. See how far you can push it. See how much profit you can make off 10 transactions.
Start tweaking the headline. Then start fiddling with the product description. Vary the photographs. Take some pictures of the thing for sale; use other photos with other items, or people, in them. Shoot really high-quality shots, and also post crappy ones from your cell-phone camera. Try every variation you can think of.
I love doing this, because there's no real risk involved. If you already have a business, you don't need to dream up a new product line or rock the boat with crazy experiments. If you don't have a business, it's a perfect way to work on your chops.