Read Aadil Palkhivala's response:
After many years of teaching, I believe that what beginning students need is not so much a meditative state of awareness as much as a focused state of awareness. While meditation is often considered a way of escaping reality (which, of course, it is not), focusing brings the student into the moment.
To cultivate this focused state of mind, it is important to open a class with some chanting, either Om or whatever mantras you know. I do this at the beginning of every class, and I recommend that all the teachers who study with me do the same. This brings students into the moment so that the external world distracts them less from their practice.
From the moment class begins, instruct your students to breathe deeply as they inhale, to fill their bodies with both breath and light, and, as they exhale, to consciously let go of the external world. It is up to you, as the teacher, to make it clear to your students that they should consciously invite the yogic force into themselves during every inhalation, and let go of the outside world during every exhalation.
If you still find that students are losing their focused presence during class, give them a posture that requires them to be completely focused. In a pose such as Vrksasana (Tree Pose), students will choose to focus rather than fall over!