I also agree with Dave/Thomas, and I request half down up front before I commit. That's after I confirm all of the details.
Be advised I wanted to hit a sweet spot, and I charge more than $50/hour, and it's never been a problem. I prefer hourly, since I got burned once on a flat rate estimate.... My hourly rate may go up soon, since I have some good news coming in a letter soon...
Regards,
Marc