I frequently used to find that I would email someone and would either need a response from them, or would need them to do something as a result of the email. Unless I remembered to keep a separate note, to check that it had been dealt with, I would frequently forget until weeks later I’d have one of those “I wonder what happened to…” moments.
To tackle this problem I set up a folder called “Follow up” and then created a filter/rule that scans all incoming email. If the sender is me, and the Bcc is me too, it marks it as read, and moves it to “Follow up”.
Then, whenever I’m sending an email that requires a response or an action, I simply add myself as a Bcc and it finds it way to my “Follow up” folder. I regularly scan through the folder (usually Friday afternoon), archive the completed items and chase the overdue (or soon to be due) ones.
NB: I know that some email clients (Outlook, Lotus Notes, etc) have a built-in follow-up capability, but I have only found this to work in an environment where everyone is using the same client/infrastructure. I don’t work in that type of environment, and, as a freelancer, I don’t always get a choice of what I use. I have found this approach is generic enough to work on everything.