A post on Vancian Crunch reminded me that I should post my thoughts on the way spell memorization works in D&D. I've actually posted the basics of this in a blog comment somewhere and perhaps on a forum, but this is a better place for it.
There's an old technique of memorization called a "memory palace" or ars memoria, in which Roman orators would find a building that was infrequently traveled, walk through it several times memorizing the way it looked, then walk through it, physically or mentally, while mentally placing symbols or objects at each distinct place in the building to represent a key topic or point in a speech. To keep the details in memory, the orator would need to mentally walk through the memory palace periodically; to keep more than one speech prepared, the orator would need to find more than one building and commit its appearance to memory.
There was also historically an elaborate system of magic using the symbols and names of the celestial rulers of planets, zodiac signs, and hours of the day to create a talisman to cause a specific magical effect. Each effect required the name of the spirit able to produce that effect, the name of the spirit that ruled the day you cast the spell, and the name of the spirit that ruled the hour you cast the spell. In the Medieval and Renaissance periods, monks and priests used the ars memoria to memorize massive lists of theological points, but several people renowned for their memory, like Giordano Bruno, were accused of dabbling in that other system, to cast spells.
Now, merge those two ideas back together. Imagine a wizard able to cast spells, not through creating a physical talisman, but through an elaborate sequence of symbols that invoke various powers. Each casting of a spell is unique, since it must include the symbols and names appropriate to the day and hour the spell was prepared. The wizard performs a ritual, placing different symbols in each position withing a memory palace and associating each with a magical word. The act of memorizing the spell is part of the casting; "walking" through through the memory palace and speaking each word in turn,with the correct hand gestures and the mental image of a specific symbol, completes the spell and releases its effect.
Thus, when a D&D magic-user learns a new spell, they are learning what symbols/names/gestures are connected to that specific effect, but this must be combined with symbols/names/gestures connected to the day and hour that the spell is prepared; this latter information is something a magic-user would have learned during their apprenticeship. The memory palace used to memorize the spell might be a physical place, or might be some astral equivalent, but the magic-user has to spend hours, days, perhaps months of walking through the palace to learn every minute detail of it before being able to use it for a spell. Each spell is so elaborate, requiring the memorization of hundreds of details, that the memory palace can only hold one spell at first. After extensive practice, the magic-user may be able to fit another spell in there, perhaps placing symbols on the left and right side of each room in the palace; later, they are able to add more spells.
However, each memory palace has its own tone and feel. The magic-user can't use a 1st level memory palace to prepare a 2nd level spell, because spells of the second level have separate spirits for each day and hour that don't "mesh" with the tone of a 1st level memory palace. So, as part of advancing to the third level, a magic-user finds a new location appropriate to second level spells and commits the details of every step of a journey through that palace to memory. Again, until the magic-user has had sufficient experience with the new palace, it can only hold one spell.
There's a lot of ways this interpretation of spell memorization could be used. For example, if magic-users need a physical location to memorize before being able to add spells of the next level, perhaps this is why they go on dungeon expeditions. Or perhaps they can't use just any old location, but must use one of several real, legendary places, which could be a spur for adventures.