It might seem obvious, but be friendly and I mean to the office staff, the TA's, the teachers. The TA's and other staff members will inform the teachers and the senior staff what they think of you! Supply is a great way to earn some money, do some teaching but without all the admin and rubbish that goes with being a full time class teacher and you can have a life outside the classroom. But you are a visitor and it's someone's class, bear that in mind.
Best course of action is to be firm with the children, check the behaviour policy but stay calm at all times. What I mean by that is with your own class, the kids know you; you can afford to get cross etc with your own class. But don't go down that road with kids who don't know you. Just keep calm, use humour, praise and positivity. Tell them they're great all day, and you are likely to have a good day. In conjunction with this, sus out the school. Some will like you to be really strict, some will like you to be warm and fluffy. Work out what they want and just go with it.
It can be challenging. Some schools you will never want to set foot in again, some will be ok, some you will like, but just go with it, so to speak. Most schools will be happy with you, just as long as you can adequately cover the class for the day without any major traumas.
The joy of supply is that if you have a bad day (and you will!), just tell your agency you don't want to go back there again, and that's that, worry no more and enjoy your evening. In the schools you like, make an effort in all respects, mark the books, praise the school and they will call you back.
And finally, always have one of those pens to hand that has all the colours. Some schools mark in blue, some in red, some in green! I used to bring a pack of spare glue sticks, pencils, whiteboard pens etc (just a few spares) as you can end up with kids without resources in a few schools and nobody around to ask.