When I used Mail, I had a lot of smart mailboxes, but one of the most used ones (and their main conditions) where:
UNREAD (message is unread)
TODAY (date received is TODAY)
YESTERDAY (date received is YESTERDAY)
WEEK (date received is within 7 days from now)
MONTH (date received is within 30 days from now).
FLAGGED (message is flagged)
And every year, I’d create a (new) one like this:
2010 (message is received between 1/1/2010 and 31 dec 2010).
2009 (like 2010, but with the correct dates)
Those ones were inside a folder. It allowed me to group messages by year, because sometimes I needed a message that I got in 2008…
Of course I’d add other conditions like not in spam/trash, etc. Sometimes you have to “nest” smartmailboxes. Like have one that is message is not in spam or thrash. And then a new smart mailbox that says: Message is not in + other conditions.
The reasoning behind all this smartmailboxing is that once you start getting thousands of emails, searching in mail is not fast. For that reason I switched to Gmail, but that’s just my personal preference.