Acting as a bill collector can be a major drain on the time and energy of a recruiter. Unfortunately, many clients may choose to pay certain vendors last especially if they think they have already received the service and have nothing to lose.  This situation doesn't improve during harsh economic times, it only gets worst.  When a recruiter wants to increase the chances of getting paid on time even in the worst of times there a few steps he/she needs to take.
1.    Put yourself in the client's shoes.  What could motivate them to pay you quickly? 

a.    fear of loss
b.    incentives
c.    guarantees

Most people are greatly influenced by fear of loss (punishment), some type of incentive (reward) or guarantee. 
2.    Ask yourself how you can use these motivators to influence your clients to pay you quickly.
3.    Here are a few ideas

a.    fees and penalties for late payment
b.    discounts for early payment
c.    guarantee client's satisfaction if they pay the invoice within a certain amount of time

Put all of your agreements in writing, invoice immediately and remind clients of any rewards or incentives you may be offering for early payment.  Don't mention the punishment (fees, penalties) until after the client is late paying.