UPDATE: The steps explained below don't work with CRM 2013. However, Scott has a blog post which shows you how to get it working again:
One of my last posts was sharing how I use Visual Studio to debug CRM web resources:
This approach was one of those discoveries that makes us developer types get a little excited because it allows us to do something we do repeatedly in a much more efficient fashion. I just came across another one of those EUREKA moments thanks to Scott Durow (@scottdurow) and some his promising new http://www.sparklexrm.com project.
BTW, Scott’s the man behind Ribbon Workbench for Dynamics CRM. It’s the best Ribbon editor out there in my opinion. I digress…
I was reading through http://www.sparklexrm.com/s/Tutorials/SetUpNewProject.html and discovered a little gift from Scott at the end of the walkthrough. Steps 9-14 explain how to use www.fiddler2.com to let you debug changes to web resources without ever having to deploy them to Dynamics CRM. How? Steps 9-14 explain it.