[gesman]

No penalties. Diplomacy. Forcing penalties is acknowledging that you lost. Lawyers love that everyone-lost-lawyer-wins attitude.

The best way to adjust client's "too-busy-for-me" attitude is to push client's weak spot.

It could be delaying deliverable (until fully paid), it could be disabling features (until fully paid), it could be threat of blogging about client's ability to pay (scares the shit out of clients who are thriving for public impression of success), it could be requirement to pay upfront 50%-100% of contract cost.

The key is to be subtle but clear without threats or pressure by presenting your game as business as usual, giving client clear and easy way to solve all issues by paying.