Require a hefty retainer up front. The retainer should be applicable against final invoice. This means you get a check upfront, then bill against the work you've done on a regular cycle and get checks for that work, except at the end where the money you got paid upfront covers the last part of the job.

The idea is that instead of the client always owing you money, you always owe the client work. The goal is to avoid people who don't pay. Good clients recognize that you shouldn't be assuming the financial risk of their project. Good clients are also ready to write checks.

A potential client who isn't prepared to write a retainer check upfront hasn't properly budgeted for your services. Often, they never will. A potential client who isn't ready to write checks, isn't ready to start their project.

Good luck.