I'm curious to know how other users organize their archive folder and any best practices for keeping it clean over time.
I think the answer depends on how many programs you run, how often you run programs per week/month/quarter, and how often you'll need to reference and lookup archived programs. I recommend creating the same or a similar folder structure you use for your active marketing programs prefixed with an underscore (i.e. _Webinars) and then creating sub folders with date/timeframe indicators (i.e. _2015 Webinars, _2016 Webinars).
Archiving old programs is always a good thing to do. You can choose to do this on a quarterly basis or half-yearly basis, depending on the number of programs you create.
You can use a similar folder structure as you are using for active programs when archiving. It would make more sense to have separate archive folder under each type of program, rather than archiving everything in a single folder.
I agree with Alok on having separate archive folder under each type of program. I break mine down by:
I archive annually and deactivate any smart campaigns in the archived programs and remove any subscriptions on any reports in the program that are no longer needed. Old landing pages contained in the archived programs are also deactivated . So I time everything before I go on my vacation at the end of the year. When I come back from vacation, I archive the images and other files in Design Studio that are no longer needed. My system has helped me find the things that I need, if need to ever clone them, look for stats, or my boss can easily find what she needs while I'm out.