People use Outlook in many ways that it just shouldn't be used. There are many of us (myself included) that use it as a "cover my ***" resource. That's where you save all of your emails just in case someone comes back and says "Why did you not tell me that 'X' was going to be thrown away??" That's when I do a little think I like to call "Advanced Find..." tool in Outlook to pull that specific email that I sent that person two years ago reminding him that we were throwing away 'X' and if he wants us to keep it, he needs to let us know.
Some of us use Outlook as a de facto database resource where they store strings of email messages on a certain topic, all within a sub-folder of Outlook. Say, for example, that I have a folder called "Passwords" where I put emails that contain all of the different usernames and passwords for all of those different databases I have access to (not that I do... but someone might do something like that after IT told him not to write them down on post-it notes and attach them to the underside of their keyboard.)
And, of course, some use Outlook as a file storage system. Instead of saving that attachment in the proper folder in the firm's document management system; it is placed in a folder... just in case we need that file somewhere down the road.
All of these are bad, bad, bad habits that everyone knows are slowing down Outlook, but they are all too set in their ways to stop doing the things that are bad for them. Besides, they argue that IT doesn't understand that it would take them days, or weeks, or months to undo all the bad things they've done. Although IT might be able to start changing the ways people use Outlook and get them to be better about using the DMS, deleting unnecessary emails, or finding other resources to use for communications, but these are habits learned over many years, and won't be unlearned without a lot of coaxing/forcing, and changes in the way that Outlook itself works.
I'm afraid that IT is just going to have to take this bull by the horns and start by changing the way that Outlook works, and then start working on the end users bad habits. Using tools like Recommind's Decisiv Email, or a nifty little tool I saw yesterday from InfoNgen called Email Prioritization [PDF], where they tag emails and move them to more 'appropriate' places, such as a DMS, is a nice start. Give us some social networking tools to have conversations outside of email. Implement some rules, such as retention or deletion policies, that force us to either move files out of email or delete them... or, if necessary, do that for the user that won't do it. Their clients already do it... it's time they got on board with those policies as well.
The process for IT is pretty simple. First change Outlook's behavior... then change the user's behavior. Yeah, good luck with that, buddy!