Have an interview plan. What is the purpose of the interview? Don't be wishy-washy. Your two questions today could have used a lot more detail.
I do many of these (and am an expert guest) on my blog at http://www.salestactics.org. Here's an example of several: http://www.salestactics.org/sales-expert-interviews/
Write out your questions beforehand. Of course one or two new questions will occur to you while conducting the interview; ask them, too. But have your base set up.
Think about your request from their perspective: WHY would they want to participate? Yes, people want to be cooperative and helpful. But No, they do not want to create content for you (for example, I am asked to be interviewed all the time by people who are compiling such expert interviews to then SELL them...why on earth should I give them content so they can make money with it?!).
So be clear about your purpose.
You really must be looking for expertise, not digging for a job. Fake purpose will be seen through immediately and it will hurt your reputation.
Collecting a series of these interviews online around a specific topic is a great differentiating factor for job hunting, however. This is positioning.