Construct your billing structure with a mind towards what that first customer invoice will look like. Will it have loads of hours per role where the customer has no idea how to verify it's accuracy? Will they see monthly units being charged for services that haven't started? Will they be charged a monthly unit for September when work actually starts on the 25th? Before you despair, remember that nothing prevents you from structuring your payment schedule so that the first invoice just says "Signature of Contract".
