Also, the same here as many have posted.
We maintain saved files of SU and LO for archiving and reference as the project moves through project phases and revisions within phases.
Our revision tracking is based on the following Project Phases:
P0 - Programming
P1 - Pre-Design
P2 - Schematic
P3 - Design Development
P4 - Contract Documents
P5 - Bidding + Permit
P6 - Construction
P7 - Post Occupancy
And, then we then append a sequential Revision number:
ie: P3.00, P3.01, P3.02 & etc.
We also note this Phase/Revision number on our LO (printed) drawings for reference.
We also find as @gsharp notes, a folder with imported files and a second folder for exported files to be very helpful in keeping track of consultant/client exchanges.