I have a set of folders (under the inbox if it's a pop account or there is space on the Exchange server, or in a separate PST if not) where I file everything, sent and received, from that person or company. If I am filing e-mails that have been sent to two people I put a copy in each folder. I don't have anything left under my 'sent' items.

I also make good use of the follow-up flag in Outlook.

Bethalize
Peer Moderator