The most difficult question in a job interview is this:
"What was the biggest mistake that you made in your last job? What could you do to prevent making that mistake in the future?"
This is seemingly a no-win question for you, since the interviewer wants you to say something negative about yourself!
There is a good answer, however:
"The biggest mistake that I made in my last job was agreeing to a group decision-making process in hiring a new employee. I knew I had misgivings about one potential hire, and yet because it was a group decision, I relented and accepted the will of the majority. As I had feared, the new employee just wasn't suited for the company.
"Next time, I would avoid a group decision-making process when hiring a new employee, since I really need to be in a position to follow my instincts on new hires."
This answer acknowledges that a mistake was made, but makes you look good anyway! The interviewer will love it.
The next most difficult question is, "What salary do you require?"
It's important to remember that your interviewer wants to hire the best candidate for the least amount of money. You may desperately want the job, and even be willing to sacrifice some pay to get it, but don't be so quick to sell yourself short.
Know what you're worth. Do some research into similar positions in your region of the country, and have the documentation to show the approximate salaries of similarly skilled employees in similar positions. Use these facts to bolster your own salary demands.
Keep in mind, however, that there are other forms of compensation than purely monetary: consider extra vacation time and benefits in the overall compensation package when assessing your salary requirements.
Knowing your value as an employee helps the interviewer to understand your value, too. Interviewers like it when an employee knows what he or she is worth. If you undervalue yourself, or if you're overly modest, that may be perceived as weakness or lack of confidence. An interview is no place for modesty!
So do your research, know your salary requirements and stick to your guns!