First Doug, don't let them get past due.  Set your perimeters when you first start working with a new client.  If they are a new business ask for a small upfront retainer.  I ask for $2000.  I figure if they balk, then there is probably going to be a problem down the road, if they say sure, I usually say, listen, it is short term until we get established with each other.

The only "trick of the trade" I can pass down to you is that I did have a scumbag title agency that tried to hold my closing fees hostage AFTER the closings were done, at the agreed upon rate.  He wanted us to keep closing his files while he was racking up the bill and not paying me, which slightly backed up our closers getting paid within 5 days of the funding date.

Knowing that he was a proud, scumbag, I hit his pride with a ton of invoices sent to all departments (accounting, sales, closing, funding, etc.) that has in LARGE BOLD TYPE:  "Sorry to hear you are having financial troubles.  Please call me so we can work out payment terms.  I hope your business will be back on track soon so you can pay your vendors and your staff."

I was paid within the week. 

^{(And hey, scumbag, if you subscribe to this board, you know who you are.)}