Hi OTR,
I'm feeling your pain here. In the last couple of years I've changed my policy: previously I used to provide a 30 day invoice with the master (fool), but got burned once by a band and once by an indie label who took forever to pay, and there were emotional complications.
The new regime is that if it's a band or artist footing the bill they agree a fee in advance and pay the full amount up front - if it takes a little longer/is a bit more complex than projected I'm not going to worry too much, and I'm now pretty good at fair estimations of the probable job cost. With labels, if I've dealt with them before and they're OK it's 30 days with rigorous credit control, and if I don't know them it's either payment up front or on delivery of master depending on how cynical I feel about them.
Business-wise, life is much improved: if a band takes two weeks to come back with approval on the ref. for example, I've already got their money and eventual payment is no longer delayed by that period. The other thing I've found is that no-one has had any objection to payment in advance, possibly because most people budget for mastering anyway and it makes no difference to them whether they hand over the cash now or later. Mutual respect has been restored, and (so far) everyone wins.