I have a pretty good handle on my email inbox. One of my best habits I have adopted is that when I’m processing my email, if the email response will only take 1-2mins to reply, then I MUST reply there and then. Simple. No need to procrastinate, flag it, or move it to a heaving folder never to be seen again.
And for a bonus habit: it helps if YOU write better emails in the first place. Make your emails as short as they can be, so that the email subject can read and reply in the same vein.