I've made several comments over the years recommending real estate investing as the most powerful form of passive income. The reason it's so powerful compared to other passive income sources such as stocks, blogging, or bulk candy vending is: there are six ways it makes you money.
A very wise man and once taught me that the key to marketing is not what you say, but how well you can transfer emotion to your audience.
I find it interesting that wisdom is often clearly laid out before everyone in a book or a lecture, and few truly understand it. It's just as Napoleon Hill wrote in "Think and Grow Rich" when he hinted that one needs to re-read the book several times; and even then, the wisdom can only be captured when a person is ready to hear it.