The first thing every church needs to understand (and believe) is that they're not immune to financial misconduct. The "We don't need to worry about that here" mentality is the reason why so many churches have been the victims of embezzlement. Even if your church isn't being victimized (right now), implementing a system of internal controls will ensure that your finances aren't at risk in the future.
Here are a few basic principles pertaining to the implementation of internal controls:
Internal Controls = Good Stewardship
Even if you trust every leader at your church (every usher, every money-counter, and so on) you probably wouldn't feel comfortable giving all of them a copy of your house key—or access to your bank account. Even if you trust them completely, there's no reason to provide such unlimited access to your possessions. In the same way, no one should be given unlimited access to the church's assets.