A nice list of Gmail tips; I especially like the Gmail search operators summary:
Search operators in Gmail work much the same as they do with Google. Type the operator followed by a colon, and then your search term (e.g., to:tips@lifehacker.com). Likewise, you can exclude terms from your search with the hyphen (-).
The search operators allow you to limit the scope of your search to the to (to:), from (from:), and subject (subject:) fields. Easy enough, right? You can also search based on labels (label:), emails with attachments (has:attachment), and even dates (after:/before:yyyy/mm/dd).