When looking at Minecraft pictures, I see that lots of people build lots of really neat structures, but I really wonder how they build some structures. Are the using some external tool to help them plan?
For example, let's take the case of a simple sphere, dome, or arch. How do you figure out where the blocks should be placed? Do you simulate the structure in some other tool? Do you use a calculator and the appropriate equations?
For simple structures trial and error would probably be an adequate solution. After all it doesn't take long to hack out any incorrectly placed blocks.
More complicated structures will require more thought and planning, but by taking things in stages you can break the problem down into more simple steps.
As for an external tool - pencil and squared graph paper might be all you need!
Just thought of something else - if you use sand or gravel as scaffolding then it's relatively easy to remove the excess material when you've finished. Just stand at ground level and hack away.
I generally build a foundation for the structure out of gravel or dirt. From there, I slowly build more of it, and tweak as I go.
I try to build the important parts of the structure first (the skeleton of it), and worry about details later.
Minepedia has many map editors that provide 2d or 3d interfaces for rapid map editing. 2d and 3d brushes (rectangles, circles, spheres), copy and paste, block search and replace, and other functions are available.
Full Glass Empty Clip has a great article on using Google sketchup to model your objects in proper 3D before you go off and build. There is a template linked in there with all the Minecraft blocks in and instructions to get you started. Not sure if you can export directly into a schematic to load into MCEdit yet, but your need for that depends if you want to build it by hand or not.
There's a map generator that specifically does that called Planetoids. It was featured on the Yogscast:
If you want to make circles, spheres, or toruses (donuts) by hand, use some pixel templates like this one:
To build a sphere for example (one method of many), pick the diameter you want and build up circles as layers. For example, if you want to build a sphere with diameter 21, you would
//hsphere glass 10
More info here.
Use wood for scaffolding or for your base, it will be easily burned and will be little effort.
If you want to make perfect spheres, the 3D plans are available in this Minecraft Forum thread. I've built some cool stuff with these!
For obvious reasons I'd suggest using the Minecraft Structure Planner. It's free, and you can download it from http://minecraftstructureplanner.com
I make my own tools, such as my voxel sphere generator.
I have found that plotz.co.uk is the most commonly used website for large scale ellipse, sphere, torus and tower building on Minecraft, and it has adjustable dimensions. I used it to build a 254 diameter death star on MC X360