Testing and correcting accessibility problems throughout the development process is all about tools. Here are tips I've collected:

• Support the fewest back versions of JAWS you can: just as with browsers, supporting earlier versions makes development infinitely harder. Current version is 11.
• Fangs is a free screen-reader emulator, a Firefox plugin: http://www.standards-schmandards.com/projects/fangs/
• Study Juicy Studio (http://juicystudio.com/) for Javascript accessibility fixes: they've figured out tons.
• Use Juicy Studio accessibility plugins and toolbar: https://addons.mozilla.org/en-US/firefox/user/320.
• WebAIM now has an accessibility toolbar: https://addons.mozilla.org/en-US/firefox/addon/6720.
• There's also a free Web Accessibility Toolbar for IE: http://www.paciellogroup.com/resources/wat-ie-about.html

Finding more tools:

• WAI lists of all manner of accessibility tools (many of which are commercial): http://www.w3.org/WAI/ER/tools/Overview.html
• Juicy Studio lists quite a range: http://juicystudio.com/services.php
• SitePoint wrote up "12 Tools to Check Your Site's Accessibility": http://www.sitepoint.com/blogs/2009/07/06/site-accessibility-tools/