Smart collections can be enabled with the config option $allow_smart_collections which is disabled by default.
This will add a new option to the Actions drop-down at the top of the Search page - "Save search as smart collection" which when clicked will create a new Collection for your user. The collection is automatically named based on the search that was performed previously and will automatically add and remove resources from the collection when any resources in the system match, or cease to match, the search criteria.
Asynchronous smart collections
Enabling the config option $smart_collections_async will cause the collections to be updated after performing each search rather than constantly in the background.