In 2012, the Purge Unused command has gotten a little help. Now, it is possible to delete out of your project all the unused materials and property set definitions. I really like the ability to delete out all the unused materials.
One thing to know about this new ability is that it may take up to three purge Unused to fully clear everything from a project. Allow me to explain. When you use Purge Unused, you only see the items that are not being currently used in the project.
Any families that are listed in the list may be using materials that are in the project. The materials that those families are using won't be shown in the list because they are "in use" in those families that aren't being used.
Confused yet? Good.
When you purge out a family, its materials now might be unused in the project and the next time you launch the purge unused dialog you may see the additional materials that can be purged from the project.
When it comes to property sets, the same thing that happened with families can happen with materials. Property sets may be "in use" by a material that isn't being used in the project.When the material is purged, its property set is no longer being used and will display in the Purged unused dialog when you launch it again.
So, long story short, you may need to do a Purge Unused three times to get everything out of the project. Once to get everything you normally would, then to get any additional materials or property sets, and finally, to make sure all the property sets are taken care of.