Driddle said:
How many DMs out there are checking their players' character sheets during the game to make sure spellcasters aren't fixing their spell picks as they move through an adventure?
Especially as you get to higher levels, one of the problems with Vancian spellcasters (i.e. Cleric, Druid , Wizard, not Sorcerer or Bard), is it can take ages to prepare a full list of spells memorized. Particularly if you're trying to roleplay time fairly fast, it's no fun to sit down and wait to the spellcaster players to memorize spells for an hour or two.
Trying to speed things up, can lead to the "but I would have memorized that" scenario - where spell preparation hasn't been done (or at least hasn't been completed). In some cases it's reasonable, party is travelling outdoors for a day or two - Leomund's Secure Shelter to pick a random somewhat appropriate spell.
My suggestion is this, get the players to write up several alternate spell lists for different scenarios:
buff for encounter
covert
dungeon
urban
wilderness
This should really speed up spell memorization. In general, they should be picking one of these lists, and making only minor changes for spell preparation.
Another thing spellcasters can, and probably should, do is not memorize spells in all their spell slots. Ok, it turns out you need Leomund's Secure Shelter - well if you left a 4th level spell free, you can sit down and memorize the spell, and then cast it. However, this doesn't help you for combats.
Another option is to switch to using spellcasters that don't memorize. This really simplifies thing for a spellcaster - no preparation, just choosing what spell to cast in a given round.
I also like Tortoise's idea. Kind of like having a hand of spells memorized from a deck of spells that you have available (sorry Magic the Gathering flashbacks). It would be kind of cool to have that with the spells details, base DC, etc, and a funky picture ... haha, I fireball - pulls card out of hand - the goblins!
And Orchid Blossom's DM's rule is nice flavor to Sorcerer's "wild talents".
Just my thoughts ...