Finding an old but important email can be tricky, even with labels and filters. Google's Executive Chairman Eric Schmidt says it's best to forward such messages to yourself with a few choice keywords that you will remember, for easy future searches.
Schmidt and Jonathan Rosenberg, the former Google Senior Vice President of Products, share more such ideas on handling email overload in their new book, How Google Works.
One of their points is to make important messages easy for you to find later:
Help your future self search for stuff. If you get something you think you may want to recall later, forward it to yourself along with a few keywords that describe its content. Think to yourself, How will I search for this later? Then, when you search for it later, you'll probably use those same search terms. This isn't just handy for emails, but important documents too, like passports or insurance cards.
Check out the full list of email rules from Schmidt and Rosenberg, such as handling email from the latest message and going backwards.