If you want to remember the face of a friend, think of a time when he or she was doing something. Often, memories are not snapshots, they're movies.
I was given this advice at a time of distress, when I was having difficulty coming up with any memories, and it proved to be invaluable. I found the features of friends and loved ones came more easily to mind. I also found I could hear their voices, and it became easier to relive the most emotionally evocative events we'd shared.
From a personal standpoint, this was a real boon. From the perspective of writing, it was invaluable. Whenever a scene needs to be deeply felt or requires a texture that eludes me, I can immerse myself in memories that otherwise feel rehearsed and stale.
The same thing is true for places. While I get a thrill out of looking at pictures from a visit to India, it can't compare to envisioning a turbaned man on a white stallion riding up to my taxi in New Delhi, leaning down to get a better look at me. A photo of my grandfather's farmhouse can't match the detail and sense of being there I have when I recall climbing onto the antique tractor, starting it up, and mowing the long field where we played baseball and golf.
I am not limited to memories. Once I am into a story, I make a point of imagining characters doing mundane things like buttoning a shirt or eating an ice cream cone. This almost always segues into something more dramatic, revealing who they are and, sometimes, adding scenes I hadn't planned. When I am working on scenes, I don't just take a picture or even stroll through them. I give myself an active job, like repairing an engine on a rocket ship or digging a grave on a moonless night.
Of course, research, especially when it is hands-on and active, feeds imagination, but don't stop there. Remember the adage,"I hear and I forget. I see and I remember. I do and I understand"? Focusing "I do" or "he/she does" when delving for memories and exploring scenes and characters can make your story more vivid and compelling.