i do a lot of large offices and contact work and have very few on contract. This is because a lot of facility managers wont sign them unless there is a claws in it were they can cancel the contract by giving 30 days notice. what i do is allways get somebody on site to sign a work completion form every time you clean them that way if you have to take them to court to get paid you have proof you've done the work. usually the larger companys take a lot longer to pay than small ones, even if you have them on 28 days account it might take 60 days to get paid. when you chase payment up they just usually tell you the accounts dept are prossesing your payment. But out of all my companys i clean they all have paid and i have only had to take 1 to court. (and you would'nt belive the size of that company one of the biggest fast food chains in the world).