Here's an expansion sheet for the standard Pathfinder Character Sheet (also works with both D&D 3e and D&D 3.5, no conversion required) I've made for my party, which includes some things which I thought would minimize the book-keeping caused by some of the unique features of the setting. In particular:
Magic Weapon and Armour Tracking: This helps track the plane of origin of each piece of magical armour and weaponry and includes boxes where you can pre-emptively record how they'll change as you move to other planes.
Magic Item School and Cancellation Track: For all the other magic items you carry, where you can quickly check which school they belong to and whether or not they are affected by a particular plane. The Null colum is checked when the item is cancelled so as to easily remember which ones are on and which are off.
Alignment Tracking: This is mostly a house rule, but since alignments play such a big role in Planescape, maybe someone else will find it of use. PCs start in the middle box of their respective alignment, and then move around the track if their actions push them toward a different alignment (usually should be just 1 step, unless the action was very extreme). For each step outside their current alignment, PCs get -5% experience, and a -1 to attack rolls, skill checks, and saving throws, until they decide to embrace their new alignment and remove the penalties. This requires 1 week of introspection and meditation; the PC can remain active during this period, but must spend at least 4 hours per day meditating or add another day to the process. The accumulated penalties remain in effect during this week.
Planar Distances: This is a referential chart (same as the one in the PS DM Screen) for easy checking of planar distances.
Outer Planes Chart: This one is both a referential chart for figuring out how the Outer Planes are called and related to each other and a track for the effects on Divine Spellcasters, where they can pre-emptively reccord which effective level they'll be casting spells as depending on how far from their deity's home plane they currently are (remember the distances for Divine Spellcasters are measured around the Great Wheel, and not through the shortest route).
I hope this can be of help to other Planescape players out there.
