I'm pretty sure the literal answer to my question is "re-immersion," or some other form of constant study and use of the dormant language. (Indeed, when I have done that with other languages—even for a short time—it's worked like a charm.) However, if one is not prepared to do that now, what are the fastest and best methods for re-acquiring a language one has not spoken in years, particularly re-acquiring facility with the active/production elements: speaking and writing?
(With my language—Russian—I find that I can understand about 75% as well as before, but speak only about 15% as well as before. In my case, it has been about 6 years, with another language in between.)