An observation that during the gold rush you wanted to be the person selling picks and shovels to support the craze. In other words, look for a supporting, adjacent business to a craze or boom. For example, during the early oil years, prices for a barrel swung wildly from .10 cents a barrel to 10 dollars. John D. Rockefeller found that the business of refining the oil to be stable in contrast to wildcatting. Further, he looked for waste product uses like fertilizer1.
1. Chernow, R. Titan: The Life of John D. Rockefeller, Sr. (Vintage, New York, 2004).