There's a danger that once you've replied to a thread in Gmail it can sink lower and lower down your priority list and eventually get forgotten about altogether. If you want to make sure that you're following up important messages when they don't get a response, then this popular browser extension is one way to go about it.
Head to the Boomerang download page to get it installed for Chrome, Firefox or Safari and let it start working its magic with your Gmail inbox. Right off the bat you'll notice a new Boomerang icon up at the top of the web interface. Click on this to manage messages that you've sent and to change the plugin's settings.
The key Boomerang feature appears when you're composing a new message or replying to an existing thread. Check the box labelled Boomerang this and then enter a time frame; if the email has not had a response in this time, it returns to your inbox to remind you that it still needs dealing with. You can opt for something vague like tomorrow afternoon or specify a precise date and time when you want the message to boomerang back.
With that done you can click on Send and the message shifts to the Sent Mail folder as normal. It also gets a new Boomerang label applied to it, and by clicking on messages with this label (via the menu on the left) you can reschedule them or cancel the boomerang action. You can also send messages back to the inbox yourself if you've had enough waiting.
By default, when emails are returned they'll appear as unread and with a star attached, just to make sure there's no chance of you missing them. If this is a little too heavy handed for your liking then click the Boomerang icon at the top of the screen and choose Settings; the subsequent dialog box lets you tweak these options and disable the Boomerang-Returned label if you don't like it.
The basic Boomerang plan is free, and that gets you 10 boomeranged messages per month (including returned messages and the Send Later feature that's also available), so you'll have to prioritize the most important conversations you don't want to lose track of. If you're going to need more than that, the personal pricing plans start at $4.99 per month, and you get a few extra bonus features thrown in as well.