Not "too many" computer smarts puts you SEVERAL ahead of me--here's what I do:
firrst I have created a folder on my computer that is "IAP", now it has sub-folders for the bash and other "stuff", but it also has "PMin" and "PMout".
So, I go to the IAP site (here) PM box and it defaults to PMins, go to the LAST page (to do the oldest ones first). NOTE the date of the first one and mark the box that says "Messages xx" where xx is the number of messages. put an x in that box and those messages are selected---select everything on that page.
At the bottom of the page is a drop down menu that says, "move to folder", when it drops down, you have several choices, I select "download as XML" and hit "go".
When prompted, I save to the IAP, PMin. I name the file "PMin oldest date --newest date" and save in the PMin file.
Last, change the drop down to "delete" and delete the ones you just saved. Do each page except the first so you can save recent PM's in case you need to refer to them.
Do the same with PM out---and you give yourself space again.
Hope this helps!!