We NEVER get paid late using this approach, in 8 years of doing business. Admittedly we are not doing software development but the principle works as long as you can get a warranty defined in your terms that does not commit you to doing free feature work rather than bug fixes. Shoud be do-able.
I typically say when a feature is released to testing/customer acceptance, I'm done the spirit of what I was approved to work on, and the work is both billable in full at that point, and the warranty clock of 30 days starts.