Gmail has an awesome design feature called conversations, where your inbox and outbox are merged into a single message stream, and you can see a complete back-and-forth exchange in one place, instead of having to rely on sane quoting practices or constantly switch between mailboxes. I don't use Gmail but I want the benefit of conversations.
Using Smart Mailboxes in Tiger's Mail.app, though, I can emulate conversations. The rules are simply to include all mail that's either in my inbox or my sent folder, nothing more. It was necessary for me to also switch the date received column to date sent, because timezone settings between my machine and my IMAP server were causing the messages to be shown in the wrong order.
I've been using this for about two weeks, and it has been much easier to track mail exchanges.

