Years ago, in the days when server storage space was expensive and small, I was forced to get into the habit of ruthlessly filing all of my emails out of my Outlook inbox and into personal folders which were stored on my hard drive.
Fortunately, storage capability has increased and cost has decreased over the years, so it's no longer necessary for your IT department to dole out email storage space like Ebeneezer Scrooge. My habit of filing email has served me well, however, so I've stuck with it and refined it a bit over the years.
I create a folder for each project or client, sometimes with multiple subfolders under it for various major topics. As I receive new emails, I read them, then if they don't require action from me, I file them in the appropriate folder.
For any items that I might need to reference later, I color code them by client/project so that they can be found in a "categorized mail" search folder that I've created. I only leave items in my inbox that are essentially my "to do" list. I color code the items in my inbox as well, so that I can see all of the ones I might need for a particular project at once in the "categorized mail" folder.
If I need to find an old email, I go to the folder for that client or project, or I search from the top level (inbox) and find it. I don't like to keep hundreds of emails in my inbox, I just don't function well with it. I don't like to have a lot of papers on my desk for the same reason (and I file those just as ruthlessly). I find the clutter of papers or an overly full inbox to be distracting.
I can also color code my meetings on my calendar by the same categories, so my emails and my meetings for the same client/project are the same color. This makes it easy to see at a glance where I'll be spending my time for the week (this also is helpful when reporting time). Unfortunately, Outlook 2010 is a little tricky to set up with categories, at least in my experience.
I also use Gmail, which has categories, but they work more like labels than an actual folder. Any item that I mark with a category will still appear in the "all mail" view. They all still basically reside in the same location, but the views like "starred" or "important" allow you to see different subsets.