Back in my freelancer days, I learned this lesson the hard way, and I became strict about payments (half in advance, the other half required to turn over the final work). But when someone I'd known for years had an emergency, and needed their software patched over a weekend, I made an exception and delivered the work in good faith. And then they tried to get out of paying for it. I had to harangue them for months to get the check.

The lesson I learned is that it doesn't matter how long they've been your friend, when it comes time to pay, they may still try to stiff you. The value of your service diminishes infinitely after it has been performed.