Collecting only those BibTeX entries that are used in a document

Scenario: You have a large single file with all your BibTeX entries. This is convenient while writing, say, a paper, because everything is immediately available. After you are finished, it becomes a problem, though, because when you send the sources to someone else, you always have to send the complete BibTeX database with them. I’ve found two solutions for extracting only those BibTeX entries that one has actually used from a database:

• RefTeX (comes with Emacs): Invoke “Ref -> Global Actions -> Create BibTeX File”. RefTeX continues to amaze me, it has many useful features for managing references, tables of contents, etc.
  
• Bibtool (available e.g. via MacPorts)