How do contractors manage cash flow when clients take 30-45 days to pay?

You probably shouldn't have them as a client if you don't have the cash to do it. You could borrow the cash BUT you should have been factoring that interest in to your price. It's the same as taking a job requiring a fork lift if you don't own a fork lift. What you do? You'd rent it! But you pay a premium for that and you'd need to pass that off on the client. If you can't afford to rent it, then you can't do the job. Same thing with them - they require a secure company that doesn't rely on them as their primary income.