More important than how you ask, is what you ask. We've already mentioned the significance of preparing core questions pertinent to the position and questions relevant to the individual, but it is equally valuable to get a complete compensation picture and his or her position differential.
Understanding the candidate's current situation - both financial and otherwise - can help you create the best offer for both parties involved, or in some situations narrow your candidate pool. This means asking about more than just his or her base salary. It requires understanding his or her entire financial package - salary, bonuses, merit increases, stock options - as well as other benefits - health care, advancement opportunities, commute time, work-life balance, etc. Most candidates change jobs because there is something specific missing in their current position. Identifying this position differential - what they want vs. what they have - is fundamental to eventually closing the deal on your terms. As well, this will allow you to ensure you can provide job satisfaction should they be the best fit talent.
This is also the opportunity for you to be clear on exactly what it is that you want someone in the position to do and the expected work style. Some employees do their best work alone, while others need the energy of a team to accomplish great work. Know the hiring manager's preference as well as the office standard to ensure the candidate will fit the daily routine of the position. In addition, consider rephrasing the typical strengths and weaknesses question to, "What tasks do you not enjoy doing?" This may uncover areas of weakness that are essential to the job at hand.
Don't be afraid to ask tough questions. Glassdoor.com reports an annual list of the weirdest interview questions, such as this question from the 2011 report, "What do you think of garden gnomes?"*** Despite these oddball questions, Glassdoor.com most recently reported that few candidates who went through an unusually difficult interview considered it negative. For instance, McKinsey & Company was rated as having the toughest interview process, and yet 64% of the candidates rated the experience as positive.