Here's a solution from Microsoft that you can integrate into Visual Studio to minify the files automatically when you build your project.
To Install:
Download the msi from : http://aspnet.codeplex.com/releases/view/40584
You may need to restart your computer after its finished.
To Use:
Edit your .csproj file and include the following at the end of the file (but before the tag):
Now when you build your project, all CSS and js files that don't end in .min.js, .min.css will be minified (See the "Exclude" attribute to exclude other files from being minified).