This is great advice, and exactly how I've worked my way into two of my friends businesses. One of the keys is being willing to help for free, just as Jerry notes. This will go very far in overcoming the hesitance that goes along with trying out someone new. Other than the obvious monetary cost, for a business owner, it also cuts down on all sorts of other issues related to contracting and hiring such as issuing tax documents. If you quickly demonstrate value, you'll barely miss the time you donated to demonstrate your good faith.
Further, questions about your competency and expertise are much more generous when your offer is free work. You can rapidly learn skills in a more structured setting while getting some project-direction help, and expectations on your results will be low (or, in my experience, practically non-existent!)