1- Ahmadinejad is a populist politician, highly educated and obsessed with media. Unlike many other leaders, he rarely loses his control and enjoys embarrassing foreign journalists by leading the conversation in the direction he chooses, as seen in his interview with Mike Wallace two years ago. Remember your interview with him last year; you need to do more research on your interviewee in order to not be surprised like you were before.
2- Watch your past interview with President Ahmadinejad again and again. Don't use a rude tone in order to appear aggressive. Your aggressiveness as a journalist will be apparent through smart, clear, challenging questions that force him to give clear and concise answers.
3- It's his answers, not your questions, that are important. If he starts responding to your questions with his own questions, it indicates that he does not take you seriously; you have already lost control over the interview. Watch his other interviews and see how he responded to similar questions in the past. It will help you to sharpen your questions.
4- Research Iranian cultural codes. For example, They do not wear tie. For Ahmadinejad and other Iranian officials, a tie is a symbol of Western imperialism and arrogance. (Watch what MSNBC anchor says about it....)
5- Don't ask cliché questions. He is not like President Sarkosy and will not leave the interview. As long as you are not rude, he will respond to whatever you ask -- he actually enjoys it.
6- Do not forget that you are a journalist, not a government offical like Condoleezza Rice or Robert Gates. Ahmadinejad may have made so many controversial remarks that millions of Iranians question what he says, but he still has some factual arguments. He might be tough and/or unjustifiable, but he is not stupid.
7- Remember that you are not just interviewing a person. He is the president of a country with a rich, historic culture and 70 million proud citizens. He is also a popular leader of Muslim world. Many people might enjoy it if you corner him with a sharp question, but no one will appreciate it if you interview him with a disrespectful tone. Remember, the interview is in Tehran; you should find a way to differentiate between Ahmadinejad the person and Ahmadinejad the President of Iran. This is the most difficult task for you tomorrow.