I'm also in sales and have the same need/desire to save my emails, but I've learned to not save everything.
I file the significant messages (orders, important inquiries etc.), and get rid of the not so significant ones, like small back and forth messages after the big stuff is covered
With the Outlook Add-In, filing your messages is a snap.
For Actions, the email itself becomes the Action and is removed from your inbox.
If email IS the action, you can open the original message from your Action list and reply/forward etc. that way.
For simply filing of reference material/emails:
For key accounts I have a separate mail folder for each (under Inbox in the navigation pane). Today, I have 38 folders for key accounts and one for General Reference. It takes a bit to set it up but once it's done, it's done.
(you can also add folders on the fly with the Add-In)
Once you process a message by clicking the file button, you can file the message by key account, as well as associate it with a project if there is one.
(If there is no project associated with the email you can simply drag and drop into the key account folder as well)
Finding messages in the future is simple. Either search in the account folder, or open the Project and view messages associated with it.
It's very simple, and sort of fun in a sick way.