Creating a mental rolodex is kind of like creating a shot list. Wedding photographers often will have a shot list they run through to make sure they get the images they need. It would be pretty bad to get through the formals and come to find out you forgot to get a picture with the grandparents. Landscape photographers should also have a list. When I go to a location I do have a physical list of locations and scenes, but creating a mental rolodex is a bit different. My mental rolodex stores images in the back of my mind. The images in my rolodex aren't specific locations but ideas or concepts. Very similar to feelings or moods. As photographers we are trying to capture feelings right? I don't have a specific composition in my rolodex, but when I come across a scene, it will trigger something telling me to capture that feeling or mood.
Now before you send me off to the madhouse, try looking at a photograph that you adore. Maybe it's a mountain scene or a beautiful beach sunset. What is your mood? How does the image make you feel? Take those feelings and add them to your rolodex. When you stumble across a similar scene, you will then be able to recognize the mood and hopefully trigger a "feeling inspired" composition in the minds eye.
The main point is to be mindful of how you decide what to photograph. What triggers you to photograph a certain subject or scene? Why do you find that scene appealing. Once you start adding things to your mental rolodex, you will be able to more easily recall those and apply them to your compositions. I know it may sound like crazy talk, but don't knock it until you try it.