Do arrive early. Not only is it respectful to your fellow yogis, it gives you time to settle in and get yourself acquainted with the yoga studio.
Do inform your teacher if you have any recent injuries, medical conditions, pregnancy, or anything else you think they should know.
Do drink lots of water. Staying hydrated is essential not just for yoga but any athletic activities. Be sure to drink enough water before and after practice.
Do make necessary modifications. Yoga is neither a race nor competition. Honor your body and take rest breaks as needed.
Do silence your cell phone and remove any distractions (i.e. noisy jewelry).
Avoid strong fragrances, including perfumes, colognes, and smoking before class.
Avoid leaving class early unless it absolutely necessary. Class disruptions are distracting to both teacher and fellow yogis.