A picture is worth a thousand words. So I like to use a good modeling program (Embarcadero's ERStudio is my favorite by a long shot), which not only offers lots of capabilities for doing the design itself but can also be annotated, broken down into submodels, etc. I also use Visio regularly to create documentation to help myself and others understand the sometimes complex business processes our database applications facilitate/track.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service