As someone who often looks after other people's critters, I appreciate:
-knowing where feed is kept
-enough feed etc. to actually last for the amount of time the person will be away!
-written instructions if medications are to be given, or if everyone's feeding schedule is different
-knowing where clean-up supplies are (this applies more with puppies)
-the cell number of the person gone away, or a local contact, if things go screwy (like a water main bursting, who would expect it? But it happens.)
-if you're not issuing a shed key, don't lock the shed where the feed is kept!!!
-going through everything step by step, so that between owner and sitter, someone is more likely to catch something that hasn't been explained, in a timely fashion
-having pen and paper available, for notes of any occurrences while you're away. This is both a courtesy to your sitter and a favour to yourself.
The animals themselves are usually easy to deal with. It's the human error factor that can throw a real monkey-wrench in the works!