Make your terms strictly payment on completion
As Martin posted, although we have never done it, Mrs smudges used to work for a specialist recruitment agency there fees were always + 25% this would be returned if the client paid within 7 days.
However they had to pay the full amount before the 25% was returned. As some just missed the deadline
Never be afraid to ask for your money, you've done the work, customer is happy, your contract is with the agent, it'matters not a jot that they are awaiting payment.
Darran