The NY Times has a popular article explaining how to organize your inbox. I've also adapted David Alan's "Getting Things Done" to manage my inbox, and the techniques are easily applied to Gmail. Here are some of my time savers:
Filter your mail
I have a bunch of labels to sort various reference-related e-mails. Newsletters, product information, and e-mail flyers from stores automatically go into a "News" label. When I have time to read them, I search through this label. I also have separate "Notifications" and "Facebook" labels to flag e-mails that come in to let me know that Netflix has received my DVD or a friend has commented on my Facebook page. These could also go under the "News" label if you don't want to call them out separately.
Create action labels
I have four action labels: "_Resources", "_Someday/Maybe", "_To Do", and "_Waiting", for sending items that I have to deal with later, or want to keep on hand. The underscores at the beginning of each is to keep them at the top of Gmail's list of labels. Resources are e-mails sent from friends about helpful sites, or attachments with pictures from family, and other items that I may need to reference later. Someday/Maybe is for e-mails I'd like to get to eventually, usually for things that I'd like to do, not responses. I clean these out once every few months. To Do and Waiting are used daily. To Do includes any e-mails I need to respond to or take action on. For example, someone invites me to a party and I RSVP. To remind myself, I label this To Do until the party is over. Then I unflag it. Likewise with Waiting-- if I forward an e-mail to someone and I need a response, I flag it Waiting and check on it later to make sure I received a response. This method has improved my productivity considerably without the stress of having 50 million uncategorized e-mails sitting in my Inbox.
Empty your Inbox
I also archive old mail. Gmail makes this easy with one button. You can always search old e-mail to reminisce about those old conversations, or find something you need. I definately don't spend any time categorizing mail by date or sender, as Gmail makes it easy to search under these parameters. I make it a goal to keep my inbox empty, so that any new mail I receive either needs a response and a label, or to be archived.
The key is to keep things simple, but not too simple. If you open and read an e-mail, you shouldn't have to open and read it again to figure out what to do with it. Use labels to jog your memory about what had to be done, and put the stress of managing your life onto the computer, so you can enjoy your life.
1. Basics: An Empty In-Box, or With Just a Few E-Mail Messages? Read On, FARHAD MANJOO, March 4, 2009.