I tell them to concentrate on what happened at the crime scene, that there is a film of it all locked up in their mind and we'll find a second of it and freeze-frame it. And replay it. Some people close their eyes or look away in an effort to remember, she said. I want them to focus on the face. Eyes and nose.
People remember eyes.
Then work out from there. I don't know if it's because it frightens them or what. But people almost always try to put distance between them and a good composite.