Sign up with as many agencies as possible.
Be honest with them about what you are willing to do, if you are not going to be waiting by the phone at 7am then do not accept same day bookings.
Be wary of 'half day' rates, in secondary there is a move to have most lessons before lunch, so a 'half day' can be 4 lessons out of 5.
I do secondary and FE so this might not apply.
If you get a long term post you are generally treated like a member of staff. Schools who use supply a lot will give you a pack with main rules and policies, time table and sometimes a printout from SIMS of each class you are teaching.
If you don't have access to an electronic register then you should have a paper one, if not get students to put their name on paper and ask one to take it to the office.
I have my own supply of bright pens with 'stolen from Ms Sash' to stop students borrowing pens and me not getting them back.
Put a mug and coffee / tea in your bag and pack a lunch - most schools you pay by finger or card and for a 1 day you will not have that.
Have something up your sleeve. I have a pack of cards with logic puzzles, it is useful when you get to a classroom and no work has been left.
Be prepared to 'teach' things you have no idea about.
My nemesis is music lessons, I have no musical knowledge and of course the work left will not involve the instruments.
Leave a note for the teacher you are covering with what you have done. About 3/4 of the way through a lesson I put the time and my initials at the side of where the student has got to- I write a note to the teacher saying that is what I have done. Useful when the work is, 'take notes and answer questions on p3'.
Expect work to dry up in May and not pick up again until October.
Unless you are on a long term contract when the final bell goes you hand over your ID, paperwork, get a signature and leave.