I try to use "because I said so" as little as possible. It's a lot more work, but they know why they're doing most of what they need to do, and they know why they're avoiding what they should not do.

Edit: I've noticed that making an effort to avoid "because I said so" has built up a habit of thinking about why I'm telling them to do something before I say it out loud. Often times, this changes what I'm about to tell them to do.