Economics is a tough subject. It can be difficult to explain the more esoteric concepts, especially marginal cost. As I was working with a student today, he had some difficulty understanding how I had arrived at an answer to a problem. He seemed to have an easier time understanding the concept of average total cost of production, perhaps because of the fact that it is linked to a math concept that is highly familiar to him. On the other hand, a strategy that worked well with this student was giving an example of what might happen to a specific business if specific things happened in a market. This helped him to understand the concept of a decreasing cost market.
My next steps in tutoring economics will include providing visual aids for the concept of marginal cost, and possibly the concept of average total cost.