I googled "tips for drywalling ceilings" and saw a "T"-shaped brace for holding one end of the board to the ceiling, while you lift the other end while ascending a short step ladder - the upright of the "T" should be an inch or two taller than the ceiling is high, and, obviously, that end of the sheet needs to be butted against the wall or the edge of the previous sheet. I would have provided a link, but the same site also recommended boarding horizontally and finishing tapered joints 14" to 18" wide - obviously there's some sinister connection there to the Drywall Mud Marketing Board.
Here's a comment from that site, attributed to "Anonymous":
"Working solo, avoid jack rental with 2X4 jig built on site that pivots sheet to ceiling. Think of letters H and U attached at their top corners; H is floor to ceiling brace, U raises the sheet. H adjusts for sloped ceilings."
I was going to add my own comment about finishing tapered edges only to the width of the taper, but they required too much information when I attempted to register - I don't need the goons from the mudders' union comin' around to "set me straight".
Gary, I've been thinking about what you said concerning how vertical boards are supported at all of their edges, as opposed the horizontal ones, which are supported at only three edges. It follows then that the edges of the ceiling boards, applied in the "accepted" manner, that is, perpendicular to the ceiling joists, are supported only at the 4' ends. On the other hand, the edges of boards applied parallel to the joists, while still supported at only two edges, those are the 8' edges.
This one might be a saw-off, since both methods obviously also support the centre of the sheet, but it'll give you something else to ponder.
Oh, and don't forget -5/8" for the ceiling, 1/2" for the walls, and 3/8" for areas that will be coved. :hey: And when you do the sanding, wear a dust mask.
Wayne