Newer teachers, I want to give you a word of advice that I really hope will help you and empower your teaching. When you are subbing a class for a well-known senior teacher, don't ever start with a sheepish, "Hi everyone, I know I'm not so and so but thank you for coming anyway. . ."  It's so unnecessary, and so unfair to you.

A few points about this:
1. Duh, of course you're not so and so. They know you're not so and so.  They go to so and so's class 5 times a week, follow so and so on Twitter, and wear so and so Tshirts.  Trust me, they know for sure you are not so and so.  The good news is YOU'RE YOU! And, even if you are new, even if you're only out of training 6 months, you still have very valuable gifts to offer just by being yourself. Don't discount yourself right off the bat give yourself a chance.
2. Beginning classes in this way diminishes your authority and undermines your student's confidence in you and your abilities. Never start your class, for whatever reason, from a place of weakness. Start with confidence that you are there to teach your class and you have to the tools to do that. Your biggest asset in any class is to be who you are. That gives you an inner core of confidence that makes students feel more at peace and safe with you. During class students are not reacting to what you say as much as they are reacting to how you feel about what you are saying.
3. If a yoga student seriously can't handle one class not with their usual teacher, I don't care if it's a resurrected Patanjali, then that's not even close to your issue. We all have our attachment stuff but you can't work that out for others. Attachment is something we have to work out for ourselves.
4. You are perfect in your imperfection. So you hold airplane a little too long or don't teach a complicated, convoluted sequence that leads to some peak pose where you attach your eyeball to your pinky toe. Who the hell cares? Trust that you are there for a reason and that your authentic teaching IS WORTH SOMETHING.

And now, to illustrate, a short story from my own past:

I have recounted this story to many of my teacher coaching clients as well as my teacher trainees. Years ago when I first started to teach a very senior teacher with an enormous following came to teach at the studio where I worked. One day, I had to sub for this teacher. I remember standing in the back room of this studio wondering:

A. If I could climb out the small back window without seriously injuring my spine or

B. If yogis really could make themselves disappear.

This was one of those last minute subs and before the days of mind-body instant update schedules. Those were like the days when yoga was in black and white and we walked to teach 10 miles uphill in the wind and snow 4 times a day. Anyway, no one knew I was subbing and they were expecting their teacher. So I decided there in that small, cramped little room I had two choices: I could go out sheepish and apologetic, or I could just go out and teach my class. I took a deep breath, put my hand on the door handle, and walked out.

2 students looked back at me, rolled up their mats, and left. 3 other students looked at me as if I had just punched them in the stomach, and the other 15 didn't seem to care. So I took another big deep breath (you all know the Amorosi deep breath and how big it is) and I started to teach. I taught my class. It was a good class for a teacher 8 months out of training. I wouldn't give myself any sort of medal for it, but it was a decent class.

And at the end everyone did their "thank yous" and walked happily out. I learned that day that my authentic class will never speak to everyone, but it sure as hell speaks a lot more to a whole bunch more people than a sheepish, half-assed imitation of someone else.

So my new teachers I really love you. I know, believe me I know, how hard it is to establish your voice in the yoga world especially today. But, you can do it. And you do it by being yourself and never ever even hinting at an apologetic attitude for being yourself or teaching your class. Trust the next time you are in this situation that you are more than enough, that it's ok to be nervous, and that you are there to be you. You are there to teach your class. When you do, watch what happens. I guarantee this advice can change your entire experience of teaching yoga and make it more fun than you would ever imagine!