I don't know if this is a tip anyone is interested in, but it helps me out quite a bit at work. One thing I wish was a default feature in mail clients is a view all option. If you look at GMail, it does this. Basically it allows you to look at your mail more like a cell phone call history. It shows both incoming mail and outgoing mail. I basically created a Smart Folder with the filters for items in either Inbox or Sent Items. Then I sort my mail by the flagged column. This way I can flag my emails that I need to follow up on, including mails I send but never really know if they get read on the other end.
If anyone else knows any good tips to keep me on top of my mail, I'd be interested in hearing about them.