I enjoy learning about history, but often I have a problem with keeping different historical figures separate in my mind. Plato, Aristotle and Socrates are three different philosophers, but for each of them I imagine a guy with a beard reading a book. Once you learn more about their contributions to history, you can help make them a bit more distinct. For example, instead of Plato being a bearded guy, you can say "Plato is the bearded guy that helped establish the teaching of the trinity among those professing Christianity." While this can be "linked" to Plato, it still doesn't give Plato any visual distinction over the other two philosophers, you only get a bit more information about him.
To fix this, you could go to Google Image search and find various paintings, statues and other depictions of Plato, Aristotle and Socrates, but you are likely to find pictures of old guys with beards with only a mild amount of variation. While this might do the trick, my goal is to keep them visually separate in my mind. If you wish to remember how Plato, Aristotle and Socrates were depicted, you could create flashcards saying "Who is this piece of art depicting? [...] Socrates." But if the goal is to keep them separate in your mind, there is something (I have found to be) even more effective to do:
About 5 weeks ago, I found a web site called ConceptArt.org. As the name suggests, it is a web site where talented people post concept art to be used in various movies, animated shorts, games, etc. While some of the concept art is used for an actual product, much of the art never goes beyond the conceptual period. Maybe the game or movie never gets enough momentum to see the light of day, or maybe the concept art is just a creative exercise. Regardless, the ConceptArt.org web site is a treasure trove of memorable looking characters, robots, landscapes, etc. Recently I have been going through their forums and saving pictures of anything remotely interesting I've found. I'm trying to organize them into different folders such as "male characters," "female characters," "robots," "villains," etc. As I encounter a historical figure that I don't know very well or cannot visualize easily, I go into one of those folders and find a piece of cool concept art and use it to represent that historical figure.
While it is certainly unrealistic to think of Plato as a cyberpunk samurai or video game protagonist, the purpose is to make him distinct in your mind. Associating some (even bizarre) sort of picture with someone will keep him/her distinct from other similar people. Also, if you prefer your mental image of someone to be more realistic, Flickr is a very good resource for simple pictures of people. Just search for "male" or "female" and organize them by date posted (You are more likely to get candid and "unremarkable" shots of people). Find a picture that you like and create a topic in Supermemo saying "Who is this guy? Plato." and look at it every day until you are sure you have established a solid enough mental "link" with the two. For me, it has taken anywhere from 3 to 7 days for any given picture-person image to successfully "link."
Some might raise to the objection "Shouldn't you imagine your own image of a person? Why do you have to be unoriginal and use someone else's work?" While creating your own image of someone is a possibility, I have recently begun viewing artwork, music, movies, etc. as potential mental "shortcuts" that I can remix as I see fit in my memory. Someone else has already captured a really neat idea (Picture, music, etc.) through many hours of effort and revision. When you're trying to use established concepts as mental anchors, however, the priority shifts; no longer is originality the most important thing. Instead one's focus becomes the efficient remixing of established concepts in the mind; the "cognitive extension" of the "remix culture" we find ourselves in.
Your past experiences and memories are an essential part (Some would argue the most essential) part of who you are. If you're going to efficiently keep information that makes you a better person (Whatever one considers "culture"), you might as well have fun while acquiring and storing that information by using other people's work.
In conclusion: That guy in red? That is my mental image for Plato. Why? Because it's useful and makes thinking more fun.