I'm a supply teacher in primary so am not sure how relevant my advice will be...
Have plenty of lesson ideas and resources at hand for times when there is no planning or when the plans left behind is not enough to fill the lesson.
Always mark any written work and leave a short note for the usual class teacher re. work covered, how well they've met the objective, behaviour etc.
Leave the classroom as you found it.
Follow school behaviour policies when required and also don't be afraid to use any tricks you may have up your sleeve.
Don't be afraid to refuse to return to schools if you have an absolutely terrible day (rare but happens).
Call the agencies with your availability every week. Call in the mornings too - I had a really slow start last September but it picked up as soon as I started ringing in the mornings saying I was free that day should anything come in.
Accept as much work as possible - the more you accept, the more you're offered.
Have a street map, as well as your phone/satnav in case it decides not to work - that way you can always find your way to the school!
If you can, always have enough money in the bank to last at least a month should the work slow down temporarily - there are times of year when it is slow - July and September mainly.
There's probably more that I've forgotten to include so if I think of anything else I'll post more.
Good luck - hope you enjoy it. I've been a supply teacher on and off my whole career (12+yrs) and love it.