Here are some Outlook tips and tricks that I find very useful.
Hide buttons if you want more vertical navbar space.
Customize the Quick Access Bar
I have mine optimized for TabletPC use. I can do a send-receive, undo, delete, show/hide navbar and show/hide reading pane.
Right click on a button and choose Open in New Window to open an additional window with that item type.
Schedule an item with drag and drop
Drag and drop an email or to do item onto the calendar in the to-do bar or the calendar that is showing in another window to schedule a new calendar item. If you do this with an email it will drop the email message into the memo portion of the calendar event.
Dropping on a date/time on a full calendar the event will default to that date/time. Using the to-do calendar you will need to specify the time. You can also customize your to-do bar to show more months, filter what appointments are displayed and choose to the task list by right clicking on it.
Write a new email with <CTRL>+<SHIFT>+M
When viewing an email <CTRL>+R to reply to the selected email.
<F9> do send-receive.
Here are a couple of articles with many more keyboard shortcuts:
What are your favorite, but maybe obscure power user tips/tricks?