As stated before me, just make sure the people you choose to hire are people you know and trust. That doesn't mean just hire friends (It usually turns out bad if you just hire someone because you're friends with them, and not also because they know what they're doing), but at least make sure it's someone you know you can trust to do their job properly.
Another suggestion, do not hire Event GMs. These types of GMs are the most common when it comes to corruption, and most that I have seen just sit around and do nothing (Or sit around playing with commands) thinking all their job entails is a couple events a day and that's it. Just hire a couple people that know how to script and/or do some spriting/mapping, and leave it at that. A few support-type GMs could do for in-game issues, I guess. Just make sure that they don't have any abusive commands, and monitor them closely.