What you said
Never go in with the attitude that you are only there for one or two days. If the school likes you, you may be there for several weeks. There is actually one massive advantage of short-term supply - you get a "honeymoon period" the first time you have your class and don't have the issue of pupils being able to observe you over many lessons and suss out your weaknesses.
The expert view
Welcome back to the world of schools. Of course this is a real challenge, and you are right to see supply as one of the most difficult jobs of them all. So here are my suggestions:
1. Don't expect perfection; you won't get it, and if you see yourself as a failure for not getting it, that will break your heart. Keep your expectations to a few big aims, such as having everyone sitting down doing some work.
2. As much as possible, try not to be alone; get to know the names of the heads of year, senior leadership team and so on, so that you know who to fetch if things go wrong.
3. Know the school behaviour system - what are the sanctions and when are they invoked? That way, they see you as part of the bigger whole.
4. Set detentions and attend them (if you can). This teaches pupils that you really do mean business.
5. Get other teachers (permanent ones) to agree in advance to pop in and take pupils out or set detentions themselves. If the pupils see you are part of the team, you will get much more respect.
6. Be super serious at first.
7. Put them into a quick seating plan if you can (ideally boy-girl), to show them you are in charge.
8. Have work on the board.
9. Make eye contact.
10. Send a pupil for help if you need it, or step next door. Just don't walk alone.
Tom Bennett's latest book, Teacher, is out now, published by Continuum.