The velocity in which you collect accounts receivable is critical to maintaining good cash flow.  If an invoice is due in 30 days depending on the customer, I usually make contact by the 40^th day.  If I am not satisfied with that outcome, by the 45^th day I send what’s called a 10 day demand letter threatening to put the account into collection if not paid within 10 days.  This works about 40% of the time and at the very least gets the customer thinking about giving you some priority.  By the way accepting credit cards can improve the receivable process because waiting for a check to come in the mail might never come while there is a spontaneous collection asking the customer for their credit card

For a more detailed road map go to this blog post I wrote:

http://cfo-chief-financial-officer.blogspot.com/search?q=Accounts+Receivable