The Most Common Mistakes Tutorial is a great SketchUp tutorial written by J-m@n - a long-time contributor to SketchUp community.
The tutorial does not teach how to use SketchUp; it teaches good SketchUp techniques by showing you what to avoid. It shows many common mistakes made by new users, and how to avoid them. It talks about creating models that are appropriate for their purpose; for example, a model made for Google Earth will be approached differently than one made for rendering. The ultimate purpose of the tutorial is to improve the quality of your models.
It's a must-read for newbies, and a good review for SketchUp veterans.
Visit J-m@n's page under the Tutorials link to read Most Common Mistakes.