Wow. A lot more replies than I expected. Giving me plenty to think about!
Maybe I've just been lucky, but I usually don't have problems getting paid. Most of my clients are made up of long time repeat clients (universities, utility companies, municipalities, museums/galleries and other well-established organizations). In my experience, they are all good at paying their bills (as long as I stand behind my work -- and I do). A handful of my clients are mom n' pop shops, and they can be slow to pay sometimes, so I try to keep those types of accounts to a minimum. (I discourage walk-ins by keeping my doors locked 24/7). Admittedly, my invoicing/collection procedures have always been pretty relaxed (probably too relaxed at times), but if I decide to take on a new customer then I do charge them in advance (or at least 50%) to be on the safe side. Credit terms are only extended to clients who have proved their worth.
This particular client (visitor's center) has been purchasing signs and banners from me for the past few years. Never any problems from them, and I know some of the staff from when they were employed elsewhere (nice people), so I wasn't worried. Their job was sort of unusual because they didn't order everything at once. They mostly knew what they wanted, but a lot of it was ordered in batches over the course of several weeks (which was expected) as they sold more sponsorships or found more places to put banners / wayfinding signs / etc. Then their event got delayed by a few weeks because the weather was too warm, so their last few batches of signs extended the project a while longer. I waited until the last batch of signs was ordered and delivered before billing them with their usual 30 day term invoice (probably not the best idea). But then they waited 3 weeks after the invoice was sent before submitting their first complaint (e.g. the yard sign issue and general nitpicking). So that's how we ended up with basically 2 months of no payment. The visitor's center is closely tied to the Chamber of Commerce, and I think it could be a PR problem for them if they don't pay. Not sure how this is going to play out, but I spoke with my attorney a few days ago. I have everything extremely well documented if I have to take them to small claims court. Fingers crossed!