By increasing supply of goods , and controlling the supply of money , i suppose
Don't buy anything that you consider to be too highly priced, and be sure to explain your reasoning to the seller.
Control stocking and adopt direct marketing avoiding all middlemen.
You use fear. You put a gun on the head of the owner and say: "Lower the price!"- He'll lower - "Rise the price" - He'll rise it. That's pretty much how it works in every corner of mother Earth.