Here's a very quick tutorial on how to create symmetrical elements in SketchUp. The first couple of pages show how to create symmetrical base units for a desk. I know the drawers look funny but it was to make it clear that one side is mirrored.

The last few pages illustrate a way to create symmetry in a book case with some curved elements.

You could apply these methods to many different facets on furniture design. They will save you a great deal of work, too.

Although I didn't do it for this tutorial, it is wise to create components and/or groups when drawing. If you make a component of the first base unit of you desk and then copy and mirror it, editing performed on one will be done to the other as well.

Version 5 Tutorial
Version 4