I do sky builts a lot. Here is what I do:
Make a prim on the ground, streach it to about 2 M wide, and sit on one end of it.
Go into edit mode, focus your camera on yourself, and then edit the Z-height value for the prim you are sitting on to the height you want to work at - any value up to 768 meters max.
ZIP! You ride the prim to that altitude! Don't stand up yet, unless you like sky-diving!
If the area around you at this altitude is cluttered with other sky builds, choose one 50 M higher or lower, but remain under the 768 M building limit. In general, keep at least 30 M between yourself and other sky builds, so you don't have to listen to each other's chat.
Look around, and if this is where you want to build, (IE: You're not right beside someone else's sky build) rez another prim on the end of the one you're sitting on, and streach it into a platform, 10 M x 10 M. Lower it slightly, so the prim you're sitting on is just above that platform. Now you can stand on that platform, and build.
(Actually, I have a selection of building platforms in my inventory that I use now, rather than rezzing a second prim. They are 10 M, 20M and 30M square, with a nice scaled grid on the top surface. Makes building go more smoothly, and gives me a precise 'floor' to build on.)
Rez prims on the platform, and move them into place. Link your building together as needed. Then I usually place a free teleporter inside the building, and note the coordinates of the teleport pad, for setting up the other end. (Cubey Terra's Mark IV teleporter is available for free at his store, and can reach at least 768 meters). You can set your home position here if it's your land or your group's land. You can set a Landmark here as long as the sim or parcel doesn't restrict where you can teleport to from outside.
When you're done, sit on the first prim again, delete your temporary platform, and ride the prim back to the ground. Then set up your teleporter from the ground up to your sky build.
Other considerations:
You can ride a prim higher than 768 meters, but you can't make a stable build above that altitude. You can build it lower and re-rez it higher, but if it moves and is above 768, it may fall apart.
If you want to fly to look around while above 200 M or so, you'll need a jet pack, angel wings, or some other flight assist method. Without that, you'll sky-dive back to 200M before you can fly unassisted. I usually just walk on prim platforms.
Note the altitude of the prim you're sitting on BEFORE you zip up into the sky to build. That is the height you'll enter to ride your prim back down!
I usually turn on the feature that allows me to see the edges of parcels, and I place a slender 10m tall cylinder at the corners and center edges of the parcel. Turn off that feature once they are placed. Then I send those 'marker poles' up to my desired building altitude (by editing their Z-values while my camera is NOT focussed on the pole), so I have a clue where the edges of the property are. If you don't do this, it is very easy to end up overlapping someone else's land, and getting your sky-build returned or deleted! Once you are in the sky, you can't see any other indication of where your parcel ends and someone else's begins.
