Change the billing terms. Instead of providing services and then asking to be paid -- ask for a retainer upfront that must be paid in advance. While many corporations like to play games with first classifying their agency as a "vendor" (instead of professional services), and then secondly requiring Net 60 or Net 90 terms -- this requires the agency to act as a bank to finance the client. If your standard terms are Net 30 and your clients are not paying on time -- simply explain to them that you are no longer extending credit terms due to their failure to remit timely payment. Change the terms to retainers (payable on the 1st day of each month or quarter). I had to do this with Dell and several others who were killing our cash flow by accumulating huge payable amounts and not paying on time due to overly complicated invoice approval processes and Accounts Payable delays. Remember -- credit terms are a privilege that must be earned. When the "trust is broken" you must change the billing terms.