Cuong,
Congratulations and hope you will have a successful interview.
As for the preparation, I always recommend people to ask themselves these questions:
1. How do I best tell the C.O. my love story?
2. How do I support it?
3. What evidence I can gather to bulk-up my support?
4. If I were the C.O., what would I want to know from the individual who claims they are in love and want to live in America together forever?
5. What are the issues I'm not feeling comfortable with to tell the C.O.?
6. How do I prevent and better prepare to answer them if they ask (because they will)?
7. Is my story straight? if not, why not? Can I make it right? How? then go do it!
8. Do I have everything they ask for this interview?
9. If I don't have everything, what ELSE can I show them to explain why I don't have everything?
10. WHAT else did I miss or don't remember to include in my package?
11. Do I have too much evidence in supporting my case?
12. Do I have ENOUGH?
13. What is my ultimate goal and objective in doing this journey? (The answer is GETTING her a VISA - everything else is just part of getting the visa process)

Don't get distracted.
Last but not least, Not one single item or request in your Visa Case will determine the decision of the C.O. upon your wife's Visa outcome. Think the Big Picture!
Good luck and prepare for the interview well, once you think you have done everything, do it again, check it again. Practice the questions with her!
Be well,
chuck and kim