I ran across an older post from the Microsoft SQL Server Query Optimization Team. It is a script that creates a database and some supporting tables and a job that uses a couple of stored procs in the new database to periodically check the Missing Index Dynamic Management View and pulls the index suggestion down into a log. Every time that an index is suggested by the DMV, it is logged. If the suggestion is already there, it increments a counter and logs when exactly it noticed the suggestion.
Sometimes it suggests some slightly crazy indexes, but any help with index optimization is helpful.