In the time I've been working freelance (admittedly, not particularly long), I've had only one client who didn't want to negotiate a better contract, and that's because the work was "emergency" work (things on their site were broken, and they needed someone to step in and fix them so their business could continue). I made them aware of the conditions, and they accepted them.

Everyone else (who I've actually done work for since I implemented this) has wanted to negotiate a better contract. I've turned down people who refused to look at the contract, because I feel that they're probably not going to pay sufficient attention to the project, and I would have to fight them to get payment.

This isn't about trying to make big bucks on late fees - after a certain amount of time, I would write it off as a bad debt (as such) and hand it on to a collection agency, or call in the lawyers.