Kevin Purdy uses two Google Apps plug-ins to help him manage his inbox and calendar. Perhaps they can help you too.
I am not a fan of add-ons, plug-ins, or other things that mess with my Gmail, and I don't think I'm alone. In more than four years of writing about technology, I've found that Gmail changes quite a bit and quite often, and technologies built on top of it have a hard time catching up. You can't really hold back on updating Gmail (though some Apps customers may hold back updates). In any case, when things go wrong, they go very wrong, and your brow gets quickly furrowed. For this reason, it's almost never worth building habits around tools meant to improve Gmail.
Boomerang for Gmail is where the tool gets its name, because it "boomerangs" email back to you - it returns it to your inbox, even after you've archived it. It allows you to have a clean inbox, and the peace of mind that comes with it, but to eventually get back to those matters if they remain unresolved.
The easiest example is when you're emailing somebody about something, and you want to make sure they get back to you. There's a check box between the subject and body of the email, which allows you to "Boomerang this message," on the condition that you don't hear back. You can also specify that you want to "boomerang" the message no matter what happens.
Choose a time frame, as loose or as specific ("Tomorrow 9am," or "June 15 8:15pm") as you want. When John doesn't tell you what time works for him for that thing you are asking about, the email lands back in your inbox, and you remember to get on John's case.
Boomerang also works for messages that you can't address right now, but you want to get back to at a specific time. If you'll know more about your conference arrangements a bit closer to the conference date, "Boomerang" that message from the event coordinator to a later date using the "Boomerang" menu in the upper-right corner of the Gmail page. Clicking that menu also automatically archives the message, because that's effectively what you're doing. That Boomerang menu is where you can access a link to "Manage Scheduled Messages," and set how those messages should return to you: starred or un-starred, read or unread, labeled as "Boomerang" or not.
Another little thing that Boomerang does is offer a "Send later" option on email, through a button placed right next to your standard "Send." That's one of those things I have occasionally wished for, especially when responding to email late at night or early in the morning, or when I feel like I'm bombarding somebody with messages.
Boomerang Calendar is perhaps a bit less essential, especially for those who already have an easily accessible calendar and schedule system. But Boomerang Calendar (currently in beta testing) doesn't even require a click. Anything resembling a date ("Friday afternoon," "May 19 04:00") gets colored, and when you mouse over it, you'll instantly see your Google Calendar schedule around that time, allowing you to quickly confirm or suggest another time.
When you're writing an email or replying, Boomerang Calendar adds a "Suggest Meeting Times" tool, which has the same kind of instant Google Calendar integration. Click it, then click a few different empty slots in your calendar, and Boomerang generates an email template with those suggested times filled in.
Boomerang Gmail is free for up to 10 messages per month, on Gmail or Google Apps. After that, it's $5 per month for unlimited messages with Gmail, or $15 per month for Google Apps (with group discounts available).
Boomerang Calendar is in beta, but you can request an invitation. I was given an early preview of Boomerang Calendar, but I only chose to write about them because their product has not broken Gmail, or been broken by Gmail, and made it seem like I have an amazingly long and efficient memory to my contacts. That is what I call added value.