@dave r said:
This is what I stress to my students. Although it might seem like it is taking extra time, keep things clean as you go. That means correcting reversed faces, eliminating stray geometry, purging unused stuff from files, reducing geometry, etc. If you are importing components from other programs, use the same work flow as is recommended for working with components from the 3D Warehouse. Import them into a separate SketchUp file so you can inspect and clean them to make them suitable for using in your model. Then copy and paste into your project file.