Do the work they left. DO THE WORK THEY LEFT!! This is so, so important. This is the comment I hear from teachers most often – that the substitute teacher did not do the work they left and it was a problem. The teacher will not ask you to come back if they are annoyed! If you want to be asked back as a substitute teacher, you have to do the work the classroom teachers left.
It is true that sometimes they simply leave busy work however often they leave work that needs to be done – even if it looks like busy work. Students may tell you that they have already done this work. Then tell them that it is a review and should be easy. If it is difficult because you cannot find the book or equipment needed, do your best. If you find what you need half way through the period, then have them start on it half way through the period.
Of course, sometimes it is impossible to do what the teacher left because the equipment is broken or missing – in this case, of course, just do your best. Teachers will ask for the substitute teacher who does the work they leave, who is able to keep the students in control, who the students like and who leaves their room neat. If you do this, you will become their favourite substitute teacher and will not lack for work.