If you intend to keep things this way, then don't call it a Money Back Guarantee. Instead, be really clear that users have a trial, and billing is in arrears. Something like:

  YOUR CREDIT CARD WILL NOT BE CHARGED!
  
  We collect credit card numbers during signup to
  prevent fraud. But we won't take a cent until your free
  thirty day trial is complete.

  The trial is obligation-free; just visit our easy-to-use
  cancellation page during the first thirty days and you'll
  never be charged.

Consumers know that guarantees and rebates are intentionally painful to collect. They feel more confident and empowered when you don't have their money yet.

One caveat: I've seen successful trials that are a week or two long. At thirty days (and with users who aren't very committed), you might have an issue with users forgetting about the forthcoming charge, and then getting angry when it shows up on their bill next month.