The Gmail -No Response script, written by Jonathan Kim, combs through your emails that are between 5 and 14 days old where you were the last person to respond. It labels those emails "No Response" so you can easily check if any of those require a polite followup email. E.g., "Hi Joe, Just checking in to see what you think about this. Let me know if you have any questions." (Note to PR folk: Not "Did you get my email??" times ten.)
To use the script, paste Kim's code into a new Google Script and set it to run on a daily schedule. (I love these cron job-like scripts for Gmail.) You can add the "No Response" label to your inbox section for top-of-mind prompting.
One of the few reasons you might want to use this instead of, say, a service like Boomerang, is the labels are applied to email threads you've already sent. It pretty much just works automatically. Yes, you have to still check the No Response label, but short of getting a personal assistant to handle your email for you and follow up on all the loose threads, this is as good as it gets for now.
Meanwhile, Google has shared an entire site of tips for Gmail and its other products.