just documentation within the module - you see it all the time from the 'good' code snippet sites. I also group functions and subs into different modules depending on what they do - mouse events, file handling, error handling, look and feel, db security etc.
Code:
Option Compare Database
Option Explicit
'this module contains subs and functions handling mouse events
'has been adapted for 64bit systems
'references modules x, y and z
'requires reference library a
and then within functions and subs an explanation of what it does, why and an example of how to call it
I don't bother noting which forms, modules etc use the functions and subs - if I want to know that, I can do a find.
Also when creating forms etc, before writing code I think 'is this something I could use somewhere else?' if so then I put into a module (often prexexisting these days) and try to design so I can just put the function name in the event - which potentially means no code in the form and a faster load time.
As time goes on, you sometimes find you have several similar functions - this may be a necessity or with a bit of work they can be combined into one.