First of all, if you believe in prayers, pray that you get a nice IO. That is a must because the kind of IO you get will really shape the way the interview will go.
Then make sure you know your story and stick to it. The IO is going to try to ask you the same question in different format just to catch you if you are lying. Make sure you undertsand the question before you reply. Ask him/her to repeat if you don't understand the question. This will also give you time to think. If hearing/speaking english is a problem for you, have a translator with you. Even if you understood the question in English, it will give you time to think while your translator translates.
Don't get intimidated by the IO and don't get scared. Make sure you use the right body language. Like another poster said, make sure you have eye contacts. Americans really believe in body language. For example, they believe that if somebody is lying, he/she won't have eye contacts. In some countries (like mine), avoiding eye contacts is considered humbility or respect, but not in America. So make sure you have eye contacts and speak confidently.
Lastly, don't worry too much. As long as you have a valid case, and you really believe that you fear persecution in your country, you will be OK. Just prepare ahead of time as much as possible (study your case, practice, etc...) then pray and leave it to the almighty (if you believe in one)!
Good luck! And please do update us with your case!