Use the Boomerang for Gmail extension to remind you when you need to follow up with important email threads.
It really is an awesome product. I use it all the time. It's amazing how nobody created it before.
One of Boomerang’s most powerful features is tracking responses. When writing an message, you can set Boomerang to give you an email follow up reminder if you don’t get a reply after a certain amount of days. If the message comes back to your inbox without a response, you can then react by sending another email to follow up.
You can schedule your follow-up message as you're writing the original email. If the person replies to your original email, Boomerang will detect the response, and will not send the second email. If they don’t reply, Boomerang will send the follow-up message automatically.