One of the biggest challenges with 3D-drawings is internal surfaces unintentionally formed that cause slicing not to work properly. This is using SketchUp Pro, slicing with Slic3r, and printing with a Reprap X400. Eliminating lines unnecessary to form the outside surfaces helps, but this may not eliminate all the internal surfaces which can be hard to see even with an x-ray view. It often takes deleting an external surface, looking for internal surfaces to select and delete, then adding the external surface back, being careful not to use a line that also adds the internal surface back.
This can be a huge issue when importing an STL file into SketchUP. The import seems to prefer using all triangles and thus the maximum number of lines with no attempt to see which are unnecessary, leaving it up to the user to eliminate lines.
What techniques do you have for eliminating or avoiding unwanted internal surfaces in drawings?