I work as a cover supervisor in a secondary school, so have the back-up of knowing the school and the students, and knowing/using the school behaviour strategy is.
But I also see supply teachers who come in, some of whom are better than others. The students, even the best ones, will always try and play up for an unknown supply teacher. In my school certainly, and in many schools nationally, these days students will misbehave unless you make them behave. Your personality and confidence is the key.
My main advice for a behaviour strategy would be:
Set your stall out clearly from the start - ie from the door. Make sure you are confident and it's clear who is boss.
Pick up minor infringements very quickly, eg 2 minutes in.
Also have a range of simple starters ready, that you can quickly instruct or write on the board, to get them engaged and busy immediately; then you can deal with the reluctant ones and find out what the work set is. (You may not get the work until you walk through the door, and it is not unknown for there to be no work there.)
In terms of discipline strategy. The main thing would be to always give a child a warning. Explain clearly what they have done that is inappropriate, tell them the consequence if they do it again, and be sure they understand.
We've had 5 mins on this task and you haven't started working yet. I will move you to sit at the front by me in 5 mins time if you haven't improved.
Our school policy, which is a common one, and works effectively is:
First formal warning
Second formal warning = name on board. And they have to stay behind at the end to discuss their behaviour.
If they choose to improve at this point, the discussion is short and sweet.
If they do not improve they get a sanction (demerit).
If their behaviour deteriorates seriously, they are sent out.
One of the difficulties is that as a supply teacher you will probably be expected to teach a full day with no free periods, and it is hard enough to find time to go to the loo and grab some lunch. Keeping students behind for a chat or keeping them in at break, is rarely practical for a supply teacher, who desperately needs those 5 mins to find the next classroom and look at the next set of work.
Just a bit of a mind dump there - hope some of it helps.