Schematic Numbering Scheme
I've consulted for several medical device companies.  They have a common theme in revision numbering.


Revisions for released documents were letters: A then B then C etc.  
Revisions for unreleased R&amp;D versions were numbers: 01 then 02 then 03 etc.
Then eventually the package would get released and become A.


That way, it was easy to tell released documents from unreleased ones.

I was also asked to maintain a revision history for schematics.  It was a table on the title sheet of the schematic.  It had 4 columns: rev number, summary of changes, who, when.

[update in response to the comment by @DerStrom8]
Good point.  I do the PRELIMINARY stamp too.  The color is carefully chosen to be eye-catching, but not alarming.