How do you explain a complex technical issue to someone who has less technical knowledge than you?

I usually try to relate/connect each element or point in my explanation, to something he/she can imagine.

For example if I want to sell system antivirus to a doctor, I will read about the human immunity system, so that when I make my proposal I will leave the impression that the product I am selling is essential.

The topic you choose to set as an example differs based on the purpose of the explanation; I wanted to convince the users that my product is essential, so I chose a critical topic in their field.