You have some really good answers from the others already. In addition to goals and medical history, I would like to add that it is important to ask the client 1. How much time do they have to devote to their workouts? (1/2 hour, one hour, etc). And 2. Where will he or she be doing the majority of their workouts when on their own? (gym, in their home, outside, travel for work, etc.).
This will help you prepare and design a program for them to use on their own. While getting and retaining clients is great, I feel it is also important to educate and empower that client to do the work on their own. I also do a free consult session, and have always had luck with that.
Hope this helps and good luck.