PDF version here
This is a thing I invented, a Slot Grid.
The bold-numbered rows (1–9) are slot levels.
The slots look a little a “squares” or “boxes” but don’t be fooled. They are slots.
The number inside each slot is the class level where that particular slot is unlocked for you.
This particular grid is for Wizards. I’m going to make versions for other classes later.
Say you’re a DM and your prep for some remote tower wizard says “She’s a level 10 caster with Fear prepped”.
Then just take the slot grid, cross over all slots 11 or higher, and cross over any extra slots for spells already cast that day (mage armor). If you don’t want to mess up the sheet, use hash marks on a separate scratch paper, place beads, make a wooden pegboard, anything – the point is to instantly get an overview of the slots available for a particular level.
Beside each row, I’m going to put a selection of spells for that row in precedence order. For example, the third row will have “Fly, Counterspell, Lightning Bolt” and they will be overwritten from left to right, so if the prep says that this particular wizard favors Fear, then Fly will be overwritten and she’ll be able to cast “Fear, Counterspell, Lightning Bolt”.