For a while now, I've been contemplating how I would depict the planar cosmology should I follow up the Basic Psionics Handbook with a Basic Guide to the Planes.
As I've looked at some different resources and iterations (and D&D has gone through A LOT of planar iterations), I thought I'd turn to the guy who was supposed to write the original book about planes for D&D—Steve Marsh. (For those of you who aren't familiar with the story, just as the manuscript was being readied for a 1986 publication date, Gygax was forced out of TSR, all Gygax-related projects were immediately shelved, and the book was never published. UGH!)
Mr. Marsh has two different cosmologies he uses: one he uses for D&D games, and one he uses for non-D&D games. The one he uses for D&D relies heavily on his old concept that the various inner planes are like sheets of paper in a box. I've always been enamored of the cubic structure that came from Mr. Marsh's tetrahedral structure of the inner planes (the cubic one is the one that Gary expanded upon in Dragon #73 in his Sorceror's Scroll article for that issue, "The Inner Planes." (You may recall the full color page cutout that you could cut and assemble into the cube.) In Mr. Marsh's drawing from 2005 (the first link above), I really like the idea that the Shadow and Dream planes be part of the cosmology, and that moving "upward" (to the Upper Planes) brings you closer to ideals and positivity, and that moving "downward" (to the Lower Planes) brings you closer to decay and negativity.
Based on the above, I've put together a planar cosmology that I think covers all my bases, and in a way that supports the way I think of the planes interacting. I've also put together a schematic of travel between the planes. This may morph over time, but right now, I'm feeling pretty good about them.