Take off your shoes- On cold days I allow my students to keep their socks on for the first few minutes of class, but eventually you are going to need to be barefoot. Yoga works the tiny muscles of the feet and ankles and these muscles cannot be reached when you have shoes on. Being barefoot keeps you connected to the earth and evokes memories of grass between your toes during childhood summers. Plus, if you're using a borrowed mat, it's rude to step all over it with your dirty shoes. Speaking of which-
Clean your mat-Especially if the mat belongs to the studio. This is non-negotiable. Many students do forget though, so if your teacher reminds you to wipe down your mat, she's not trying to be rude. A lot of students use a studio's mats which can lead to a proliferation of bacteria.
Turn off your phone-Honestly, I'm not sure why this is still such a problem. If you are waiting for an important call, don't come to class. It is extremely rude to answer a phone during class, but it's also rude to send text messages or check your email, things I see happening in class every day. Even celebrities do it. Jessica Biel was reportedly asked to leave a West Hollywood yoga class for using her Blackberry. Respect your fellow students and teacher-turn your phone off and forget about it until class is over.
Listen to the teacher-This covers a number of things. Sometimes my students flat-out don't do what I'm asking. It is often a problem of something not "connecting," in which case I have to modify my cuing until it does connect with the student. Often, though, it is a case of the student misguidedly desiring a better workout. For example, during Urdhva Prasarita Padasana, I tell my students to make each movement take 2 full breaths, but invariably I see 3 or 4 people doing several movements per breath. This may make them feel like they're doing more, but they're actually cheating their abdominals out of a workout by bringing the back and legs into the movement. In a mixed-level class I will give a few options per pose so that those who need a better workout can do a more advanced modification. That is fine. Doing what you want despite what the teacher is asking, however, is not.
Come in on time, and don't leave early-Period. Respect your teacher and fellow students by honoring the commitment to being on time. One of the biggest complaints teachers have about their students is that many people leave during Savasana. Most yogis will tell you that Savasana is the most important part of class, but many students have a hard time believing that. All I can say is PLEASE respect your fellow students who are trying to relax. It is extremely distracting to hear mats being rolled, jackets zipped, and doors opening and closing while you are trying to deeply relax. If you must leave early, let your teacher know before class that you'll be leaving early, then leave before Savasana.