My IMAP Sent Messages folder in Mail got pretty full once, so I turned on "Delete messages older than one month" in the Preferences -> Accounts -> Special Mailboxes pane. Of course, today I needed something from a couple of months ago.
You can compromise between deleting messages older than one month and never by editing com.apple.mail.plist in your ~/Library -> Preferences folder. Using the Property List Editor (double-click on the property list file), look under Mail Accounts for NumberofDaystoKeepSentMessages, and change it to the number of days you want-- I chose 90. You can also change the values for your Junk Mail and Deleted Messages folders, too.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030812193842164