Leave them in the 'archive' folder for the three folder example you gave. With a little bit of setup of contacts/categories and filters you can have the ability to sort by clients, administrivia, personal, etc without needing seperate folders.
I ran MS outlook from 97 up until a couple years ago. Around 98 or 99 I hit the 2G limit of a pst (outlook data file) and started archiving. As I used it more and more by 2000 I ended up needing to archive monthly to keep things under control. This meant I had a task for once a month to archive everything then reimport anything I didn't want archived to seperate archives in a folder structure seperated by year and month. Imagine the fun when your boss asked if you sent that e-mail to client X, I had to open the most recent archive then search for it, then open the next most recent and search, then the next most recent, etc. or tell outlook to open all the archives always and wait 15 minutes for it to startup and then spend 30 minutes for every search as it dug through all those folders. Then I started to follow this pattern with everything else, scan a receipt and it went into the year month folder. Wanted to find a receipt, spend 30 minutes waiting for a search.
My point to this example is archiving is bad, my beleif is now in organizing and tagging things so they can be sorted/filtered in ways that I don't need to archive or search. its taken a couple years to finally get things back from those archive folders to something sensible but I've finally gotten everything 90% re-organized to where I only have one Archive folder where things move to and stay but are organized and I don't have to do any maintenance to keep it that way.