Programmers use version control systems to track and monitor code revisions. Writers can bring the same functionality to their drafts by following Rachel Greenham’s Mac OS X Subversion tutorial:
What does it [Subversion] do? It manages multiple versions of a project in development. You check your project out of the repository, make changes and you commit those changes back to the repository. At any time you can view older versions of the whole project or of individual files, and revert to them, if the work done since was in error. You can make branches, which allows you to develop your work in two (or more) ways in parallel, and you can tag your project to say, at this point I met a certain milestone (eg: first draft, second draft, version sent to publisher X, version sent to publisher Y, published version, etc.)
(Via TUAW)