It is not just about visualizing but also the choice of visuals. One of the things that Harry Lorayne stresses on, is to visualize something out of the ordinary, so crazy that it sticks out in the memory (for e.g, to remember a sequence of man and pencil, imagine a man with pencil sticking out his head or a pencil walking like a man on the street). The second best option is to imagine something sensory, tactile or olfactory or anything visceral. Mundane items or events are the ones most likely to escape our complete attention and consequently slip through our memory. Truth is that the attention is the most important bit of memorizing. More often than not the act of processing the information registers it much more effectively than any of these details. Try it for yourself, you will be surprised.