I've LOVED what everybody else has written so far - what awesome ideas! My thoughts on how to increase your studio size will mostly be a reiteration of what's already been stated. I especially love this post because, as I'll be moving to a new area soon myself, I need all the tips I can get!
A few ideas:
1. Like Jonathan suggested, give a lecture-recital (or just a recital) in the area. Put ads for it in the newspaper, in schools, etc - get the word out!
2. Join a local music association. Find out who the leaders of such organizations are in your area, CALL them and become acquainted - introduce yourself and don't be shy about your accomplishments and what YOU have to offer as a teacher! Then let them know that you'd be interested in contributing to your community by becoming actively involved in their organization, and is there anything you can do at this point to help? (I think this is one of the most effective ones, since you'll then become acquainted with other teachers who can refer other students to you).
3. Fliers in all the previously-posted locations ;)
4. I read about this one and I'm going to try it when we move: Bake piano-shaped cookies (I already have the cookie-cutter for it!) and visit the local schools in your area - introduce yourself (and give cookies to) the principal, as well as the music teachers in those schools. Alright now, this might be a lot of cookie-baking, but maybe it'll be worth a shot! I'm excited to try it!
5. As stated before, the *most effective* way to gain more students is by having your current students refer you to others! In one of my undergraduate piano studio teaching classes, I remember our teacher told us a very effective method he uses to motivate student referrals: He tells his students "If you refer at least one student to me, then the first month that the new student starts lessons, YOU get free lessons that month!" Saving money is ALWAYS a motivator for parents, he explains, and you won't be LOSING money the first month you're teaching that new student, but you'll DOUBLE your money the next month (since now you have one more student in your studio). I hope I explained that clearly enough!