That is, purge your SketchUp models once in awhile to help keep file size under control. You should purge Materials, Styles (in SU6) and unused components and layers. These can be purged manually--there's a Purge option in the Materials, Styles and Layers menu as well as a Purge button in the Statistics dialog. (Window>Model Info>Statistics) Here's a link to a Ruby script that will do it all automatically. PurgeAll.rb
Put this script into the Plugins folder. You will find an entry called Purge in the Plugins menu the next time you open SketchUp. Click it and a dialog box will pop up asking what you want to purge. You'll also be given an option to see a report of what was purged once the script has run.