Labeling everything is a very good idea. Then when there's a disaster and your sitter calls you and you say "unplug the protein skimmer" he know's what you're talking about. I would also stress that everything has to happen slowly in a reef tank (changes in temp, salinity). I have a tank in the office that someone else takes care of and they top it off with fresh water. One time I had done a water change over the weekend but ran out of saltwater so the level was down about 5 inches. When I came in the next day to mix more water to replace it, I found that she had added seven gallons of fresh water to the tank to "top off" Grrrrrr.
Also, make sure they don't over feed - well minded sitters are nutorious for that... so am I.
I would also ask your sitter to rinse their hands with fresh water NO SOAP before puting them in the tank. I'm always paranoid about chemicals.
And show them where a net is to remove a fish should one die. Though fish deaths shouldn't happen, the sitter needs to know to get the fish out ASAP so that bad doesn't turn to worse.
Long shot: set up a webcam to look at the tank so that you can see it over the internet. Either have them show it to you every once in a while or use a remote desktop software so that you can access the computer that it's hooked into and take a look for yourself.
And, the biggest one of all, as said before, make sure they call you if anything looks at all funny.
Best of luck.