Today two cool new features have been added that will help you "keep track"!
When you visit a user's profile (if you're logged in to the site), you will see an option to add the user to your favorites right next to the username. When you click the user will be added to your favorites. There is a new "Favorite Users" item in the user menu which shows you the list of your favorites. On that page you can visit an individual user's page when you click on the username, delete favorites and best of all, see the sheets from all of your favorites in a single page.
Another little new feature is "Sent Messages". This behaves slightly differently from a usual "Sent Mail" folder: If a message is in bold, it means the recipient hasn't read it yet (that is not quite true since they might have read the email notification which includes your message and not the actual message on the site -- but nevertheless it's better that nothing as there's no way to tell if they have read the email or not). Also if they delete the message, it will also be deleted from your sent folder.