We used to have my parents look in on the cats when we were gone. They're great people and treat the cats well, but..uh...my cats are of that one-person type. When we came home, they'd be hiding for a day or more.
A few years ago we found the ultimate cat sitter. She's a gem. The cats love her. She brings them toys and catnip. When we get home, the cats are no longer hiding--it is more of a "Darn! You're here again? Can't you send the sitter, instead?"
We also try to get away every month or so for a weekend. We have the sitter come over even though it is only a day or two just to keep a familiarity. An animal behviourist said that this would help with their anxiety. It seems to have work.