When you first hear the name, John, for example, look at them and find something in their appearance that distinguishes them. Red hair, for an easy example. Then think of some ridiculous way to connect John and Red and repeat that to yourself as you look at John and talk with him. If you were a fan of the tv show The Mentalist, for example, you might think "This is Red John, the serial killer! Red John! Yikes! Red John!" The next time you see this guy, you might think he's a serial killer, but at least you'll remember his name.