By Allen Barkus, available for private lessons in the Philadelphia, Www.AshtangabyAllen.Com. I am now also co-founder of Fifth Avenue Yoga in New York.
As a yoga teacher who has been teaching teachers for many years there are some very important things I recommend you to consider before choosing a yoga teacher, both private and for going to classes at a yoga studio or gym. Ideally I recommend a studio. A good teacher understands the practice is purely about you and can guide you towards a lifetime practice to keep your body and mind healthy. A not-so-good teacher does not take the time to understand an individual student's' needs, potentially leading to injuries. Recently one of my former students whom has physical limitations trusted a new teacher at a studio and ended up becoming seriously injured, leading me to want to share these guidelines:
1. The first thing you should ask your teacher is about their own personal practice. The guideline I use before teaching a teacher to teach is that they should have 5 years of personal daily practice, not just going to classes. There is a lot to be learned from a personal practice. In Ashtanga yoga the traditional way to learn is Mysore style, a guided practice that allows you to progress only as you are ready and able.
Ideally your teachers should be experienced and follow a direct yoga lineage. My teachers were taught by Manju Jois, whom brought Ashtanga/Vinyasa yoga to America in 1975 along with his father. I started my practice in 1998. In 2004 Manju became my teacher and continues to be my exclusive teacher. Manju's father, K. Pattabhi Jois, originated the modern Ashtanga practice. Pattabhi Jois was taught by Tirumalai Krishnamacharya, also the teacher of B.K.S. Iyengar, and is considered to be the "Father of Modern Yoga".
2. Should you go to a yoga studio or gym? Having a yoga studio focused on a style that suits your needs, physical, emotional and/or spiritual is ideal. If a yoga studio is not available to you, yes there are good teachers that teach at gyms. I met my first teacher at a gym.
3. Once you do get to get to a class there are some very important things to keep in mind. The first thing is the time on your mat is purely about you and your teacher should understand that. I've encountered situations when teachers bring their personal issues into their classes, not a healthy environment for students. A good teacher understands their students have busy lives and that is their time to focus on themselves for personal fitness and healing. The second thing to keep in mind is that a good teacher will take the time to observe you and get to know you and your limitations before giving assists in asanas/poses. In one case I had a teacher at a studio that had never seen me before stand on me, something that could have easily injured me, when he could have easily given me a more gentle/thoughtful assist. The third and possibly most important thing is that you have to be mindful of your own body and not to do things you are not ready to do, an easy way to get injured. Yoga is a life practice; Every body is different and some things will be easy for you and some things harder. Therefore you should be just focused on your own practice, not what anyone else is doing. If something is too hard for you, just take a break and do child's pose.
Your teacher will explain what that is.
If you make a commitment to your practice you will be amazed about what you are able to accomplish. One of my students lost 70 pounds in 8 months just doing his practice with no other life changes. He also healed a serious breathing problem and some other issues.
4. What about yoga online videos, DVD's, and books? I personally think these sources and be both helpful and damaging without the guidance of a personal teacher since basics including proper breathing and alignment need to be understood. If proper alignment is not practiced injuries can easily happen and an experienced teacher will alter asanas/positions to your personal needs and abilities. Also if not doing proper breathing/pranayama you are doing stretching, not yoga. Therefore if not breathing properly you are not gaining the full benefit of the practice.