I found the structured interview to be the most stressful of the three sections of the Foreign Service oral assessment (unless you count the waiting for your results! More on that tomorrow!).
Before you take the exam, you will be given a list of 13 "dimensions" the service is looking for. You should look at those closely before you ever go to the oral assessment. You don't need to memorize them, but do think of a story from your life that exemplifies each of those dimensions. Write them down if it helps you remember, but you don't need to fret about having it written down and in front of you. If you take the time to really think about the list, the examples will come to you. Then when they ask about times you have been in particular situations, like dealing with members of another culture, you will be able to bring to mind an example pretty easily.
At the start of the interview, they will put you in a small room alone and close the door. This is NOT to freak you out. It is just to give you a minute to collect your thoughts. You will then have two interviewers. Trying to "read" them may not be all that helpful. They are not supposed to telegraph to you how you are doing. I tried to tell from their expressions how I was doing, and I left thinking one loved me and one hated me, and that I had either aced the interview or bombed it!
And finally, remember to be yourself. If you made it to the orals, it is because your QEP essays convinced the Department that it wanted you. This is just your chance to seal the deal.