Gmail, IMAP and Archiving
Here's a little tip if you use the Mac Mail application to read your Gmail.
You can hook Mail up to Gmail through IMAP, using these instructions.
But once you've done that, you should make sure you're using these recommended client settings, because the default is that mail that you delete through IMAP gets moved into Gmail's Trash, and Gmail empties the trash every 30 days.
From Google's recommendation page:
Do NOT save deleted messages on the server. Messages that are deleted from an IMAP folder (except for those in [Gmail]/Spam or [Gmail]/Trash) only have that label removed and still exist in All Mail. Hence, your client doesn't need to store an extra copy of a deleted message.
The key is these settings:
Trash:
Move deleted messages to the Trash mailbox > do NOT check
Store deleted messages on the server > do NOT check