Unfortunately, the larger your business becomes, the more you get strung out, the more capital you need to save back, the bigger the customer, the bigger their debts, and the longer they take to pay. The larger their accounts payable depts. is, the less efficient they are. This is a given.
The best advice here, I feel, has been from Dana 1028. Find out what your lien law is. Submit a copy of the law with your contract.
SHOW NO MERCY IF YOU DON'T GET PAID
(We've got a "20 days from last time on the job" law here in CA. I've gone back after 6 mos. to do a "free warranty checkup" call. No charge to customer, just sign work order. Gone the next day to the courthouse).