You may be tempted to delete items from the To-Do Bar when you've completed them. However, there's a better way. When you complete a task, mark it as complete using the Mark Complete command. (A quick way to do this is to simply click the flag next to the item.) Completed items will drop off of your list as you might expect.
But what's really helpful about marking an item as complete is that you'll see it listed on your calendar at the bottom of the day on which you completed it. And it'll be crossed out so that you'll know you've completed it.
It's a great way to remind yourself of all of the work that you've done.
In the To-Do Bar and in the Calendar, any task not marked as complete on its due date will automatically be carried over to the following day until you complete it.
Caution If you choose to delete an item from the To-Do Bar, the flagged message will also be deleted from your Inbox. Similarly, if you delete the flagged message from the Inbox, you'll also delete it from the To-Do Bar. Mark Complete is safer.