This is like most of the other dumbass conspiracy theories out there: it would take too many people lying for way too long to be believable. Think about it.
1. What about all those rockets shot off from Cape Canaveral? Was everyone who says they saw them take off a liar? And with a rocket as sprocking huge as a Saturn 5, much bigger than what is needed to get an Apollo-sized capsule into Earth orbit, well, that's an awful lot of expense and energy for a “fake” Moon trip.
2. Thousands and thousands of NASA engineers, astronauts, and administrators, not to mention politicians over numerous Presidential Administrations, Republican and Democrat, kept this all secret? The US government couldn't keep Stealth bomber technology and it's torture centers secret–why do you think they'd be more successful with this–over fourty years?