LGM has printed hundreds of SketchUp models over the years. The following is a list of plugins we currently like and some best practices we suggest you keep in mind when drafting in sketchup for 3D Printing.
CADspan by LGM T. - The CADspan plugin allows SketchUp users to generate 3D Printable STL files without the headache.
First and Foremost I have to mention CADspan because it is my favorite and the plugin that I use the most.
Solid Inspector by ThomThom - Inspects and highlight problems with solids.
This is a useful tool for fining holes in your model, but the usefulness depends largely on your geometry and how you drew the model.
SketchUp STL by SketchUp Team - Import and Export STL files for 3D printing.
If you have the CADspan plugin then this is rudundant because CADspan has the same functionality in a more robust package, but this simple plugin will provide you the opportunity to import and export STL files.