1. Do your best to understand both the company and the person interviewing you.
A good source of information about the company is other people who either interviewed there or work there right now. Your goal is to understand how much they might be interested in you and how flexible they could be. With regard to your interviewer, you might want to know who this person is. What drives them? What constrains them? It is one thing to be a tough negotiator with an HR person. It's a different thing to be tough with your future boss. The more informed you are, the more likely you are to have a successful interview.
2. Justify the numbers you are asking.
Nothing hurts your credibility like requests that you cannot justify. Some objective data about the market is a good starting point. What's the market range that people in this position are getting? Explain why you should get what you are asking and give good reasons. It is not just the interviewer who must be able to clearly see why the numbers are reasonable. Other people in the company must see it the same way too.
3. Negotiate multiple issues or interests simultaneously.
Imagine the following situation. You receive a job offer from a company that you like. But you are not happy with three things about this offer. So you decide to send them a letter asking to provide you a flexible schedule. Your interviewer does their best to make it happen and come back to you. And you bring up the second issue: now it's the salary and bonus that you would like them to increase for you. After they have done that, you are asking them to place you in a different city. Imagine how annoying such an approach would be.
A much better strategy is to ask for all the three things upfront, highlighting the priority that different issues have for you. If you do not let the company people know your priorities, you might get things that are easier for the employer to give you, but that might not be as important for you. You might say "I would like a bigger bonus, but I am willing to be flexible here. What I was really hoping for is that I could work in the New York office".
4. Consider the whole package. It's not just about the money.
Salary is important. However, if you focus only on the money, you might not get the most out of the negotiation. It's a lot better to look at the total value on the table. It might be possible to get more value if you are open to exploring different alternatives available both now and in the future. For example, there might be different constraints at different stages of the hiring process. So if the company is not able to give you what you want today, they might be more flexible in 3 months. The challenge is to structure the process in such a way that they are able to reward you when they can. The more opportunities you provide for the employer to make you happy, the happier you will be with the outcome of your job negotiation.
5. Be ready to answer difficult questions.
It's not a very sound strategy to hope that you are not going to be asked hard questions. In case you are asked and you have to improvise, it might sound defensive or even untruthful. Different questions might require different strategies. If you are asked whether you have another job offer, it might be not too much of a deal to give them a confident no if you don't. After all, your popularity is not the main reason why this company wants to hire you. Some other questions might be more challenging. In that case, it is important to understand why you are asked that difficult question.
Usually, interviewers are not out there to stress you out. They might be formulating the question in an awkward way but might have a legitimate concern they want to address. Your job is to figure out that concern and to address it.