I have read numerous marketing and sales books and articles. Some geared specifically towards massage and others that are just general. Every single one, without exception, suggested that you "assume the sale" and ask "when would you like your next appointment". The truth is that most people are looking for direction and to be sort of told what to do. When you say "is 2 weeks from today at the same time good for you?", you are letting the know that you think 2 weeks is a good time for them to come back and that you believe in massage and that this is good for them. They usually take the lead and will agree with you and rebook.
That being said, of course you need to be careful of your body language, tone of voice, etc. and cannot make it sound like you are some sort of dictator demanding that they come back. I have never had a client who is offended by my approach. Of course, not all rebook at that time and they will usually say something like "I have to check my schedule and get back to you." or "I think I'll wait and call you when I feel like I need to come back." I'm always understanding and respond with a very relaxed "I understand. Well, I'll be here when you're ready, just give a call or shoot me and e-mail and I'd be happy to set things up for you."
I also do not make this suggestion with every client either. If they are coming in with their holiday gift certificate and just relaxing and have never had a massage before, or the last one they had was on vacation 5 years ago, I realize they are most likely going to be one of those once in a while clients. With those types of clients, I might take a different approach and say "Can I schedule another appointment for you?"
When I began changing the way in which I asked the clients to rebook, there was a dramatic difference in the rebooking rate.
Another thing that I did differently to fill my appointment book faster was carrying around my appointment calendar and when I run into people who say "I'm going to call you for an appointment.", I take out the appointment book and say "Is Monday at 2 a good time for you?" I might get 1 in 10 who are able to make a commitment right there in the middle of the post office or wherever I run into them, but it is 1 more client than I would have had if I didn't do that. I have had numerous people thank me for doing that because they know they probably would have put it off for longer if I didn't "help" them along.
People are busy and tend to put themselves at the bottom of the "to-do list". When I employ techniques to help them get their massage appointments made, it helps them on their road to wellness, and helps me to fill my appointment book. Everybody wins.