Back her up, even when she's wrong
The fastest way to lose a sitter is to overturn her in front of your children. If she said no to the second ice lolly and you come home and grant it, you have told everyone in the room that her word is provisional. The child will test it next time, and the time after that, and eventually the sitter will be managing an evening with no authority at all.
If you disagree with something she decided, say it to her privately, later, as information rather than correction: next time, he's allowed that, don't worry. Children adapt easily to the idea that different adults run things slightly differently. What they exploit is inconsistency between adults who are supposed to be a team.
The things nobody mentions but everybody notices
Leave food that is actually for her, and say so explicitly — "there's a plate in the fridge, that's yours" — because otherwise many people will sit for five hours and eat nothing out of politeness. Make sure the wifi password is written down somewhere rather than requiring a text message. Keep the heating on. If the dog barks at strangers for the first twenty minutes, warn her.
And be honest about what the evening actually involves. A sitter who arrives expecting two sleeping children and discovers she is doing homework, a bath and a teething baby will do it graciously once. Whether she does it twice depends entirely on whether you told her in advance.
