Tips for Selecting a Web Development Firm?

cyrano asks: "The organization I work with is looking for nothing less than a complete re-launch of its web site - upgrading from cobbled together static HTML and ASP pages to nothing less than a dynamic, database-driven site with a full-featured Content Management System and a secure eCommerce component. I have already collected proposals from several firms, each advocating the benefits of Java and Struts vs. ASP.NET vs. PHP...however, the technology used by each firm will only form a small part in my final decision. My true concern is ensuring that the firm I contract will be professional, cooperative, timely and will ultimately deliver their services as promised. What sort of questions should I be asking them, and what sort of warning signs should I look out for to make sure I find the perfect fit?"