This should be a two-phase approach.
First phase, get them to talk:
• “What about your position do you like?” When they reply, ask another question to get more specific information.
• “Yes, I can understand that. The best performers are usually happy with their positions. I think good people and position satisfaction go hand and hand, don't you?”
• “How long have you been with the company?”
• “Yes, it sure sounds like your company is taking care of you; but what if you had an opportunity to enjoy all these advantages plus ....” Mention whatever has not been brought out as a plus or negative.
• “Don't you agree that if you had the same advantages (describe them) at another firm, plus (describe other advantages), it might make sense to look into the opportunity?”
• “Great, my client is only interested in the most qualified candidate, and since you are satisfied with your current position you wouldn't qualify. Do you know of anyone who would qualify because he/she may not be as satisfied as you?”
• “That's great. I'm sure you agree that you are in the minority.” Wait for the yes, then pounce, “who do you know that's not as fortunate as yourself?”
• “If I do run across an excellent opportunity in your area, would you want me to pass it by you?”