Signing a contract is useless.....too many people have used and abused that method already. It's simple...stick to what you're saying and you'll be fine. Don't just tell the girls about your rules, enforce them. When the girls see you're serious, they'll listen. You'll always have the odd rebel, but that's the best time for you to catch them and prove your point. Send in your friends once in a while for a massage, or look through the peephole.....those are your easiest options. Also, getting close with one of the girls without the other girls knowning, is also a great way to stay in the loop.....they always talk amongst themselves.
And lastley, don't kid yourself, the LE knows exactly who's doing what. They know who to bust and who not to bust. So stick to your rules and you'll be just fine. Just make sure you get all the preliminary work outta the way first (ie. ensuring the license can be renewed, etc.).
Good luck!