I have always been very fortunate with getting paid on time. I require all clients to provide a 50% deposit in order to book a shoot with the balance due upon receipt of final images (in reality, I give clients up to 30 days to pay that final bill and once in a while, an invoice has gone to 45-60 days). In nine years of shooting professionally, I have never had to write off bad debt.
In my years as an ad agency account manager, I remember the "we don't pay our vendors until our client pays us" rule and despite my disagreement with it, I did have to abide by it when I hired photographers to shoot for my clients. However, I felt then and I still feel now, this rule weakens vendor relationships.
I pay my assistants, accountants, book keepers, web designers and other people who provide services to me within 1-2 weeks of receiving an invoice. As a result, they all jump for me when I need them, which allows me to provide better service to my clients. By paying my vendors in a timely manner, they know I respect the service they provide.