Keeping it simple.

Call me sad if you like but this is the way that all my kits are kept prior to building.
These examples are of Kitmaster Mk1 coaches- I have several that arn't complete so they are stored in boxes with labels. As parts are added they are crossed off the label and eventually I have a complete set of parts to complete a kit rather than have lots of half built efforts hanging around.
This I do with all my models, nothing worse than starting something on a cold winters night to find that you have no wheels etc.
Just an idea that might help someone.