I am a new and enthousiastic user of 3D printing. I am awaiting my first design from Shapeways.
For the time being I'm using Google SketchUp for the design, with a plugin to export to a .stl file. I inspect the stl file with the program VisCAM View, and make sure that at least there are no flipped triangles or unmatched edges in my model. And I try tot eliminate overlaps and intersections also, but that is not always possible.
Still discovering the possibilities of SketchUp with the various plugins.