Subject tagging: recommendations for Dryad curators and scientists

How do scientists tag their data? Do they come up with their own topical terms, do they consult controlled vocabularies, or do they use some other source for these terms? The goal of this project is to write a memo or guide which will help Dryad librarians guide scientists to describe their data in the best way possible by submitting meaningful subject headings/topical headings with their data.