The main thing is to be a good conversaltionalist and a good listener. Find out as much about your interviewers ahead of time and listen to what they are saying to you during the interview.
Ask them how long they've been there, what's the best thing they like about the school, where they've worked before, how they got into their field, what direction do they think school is headed, what needs to be improved about the school, etc.
You can also ask them questions like how much of the class is already filled, but I would not word your question as bluntly as you proposed. You could always ask something like "What characteristics do you feel the admissions committe is most interested in finding in a successful applicants"? Or something like that.
You can let them know how serious you are about the school by closing with a statement of around 200 words that summarizes how well you'd fit into the program (based on what you've gathered is important to them) and how the school is a very high (or top) choice for you. Be absolutely sure that you know specific numbers about the school, the mission statement, professors research interests, any special programs, etc. Weave into your closing statement how much you like that they stand for and tell them how you are on the same page.
It's all about connecting with the people you're talking to.
If you have time, I highly recommend the audiocassette "101 Answers to the Toughest Interview Questions" by Ron Frye.
Good luck!