I ran across a great article today that gives a thorough step-by-step tutorial on how to turn on compression of ASP.NET files in IIS6:
Enabling HTTP Compression in IIS 6.0

I was actually really surprised that this was not as simple as clicking a checkbox.  The IIS Manager does have a checkbox for enabling compression on application and static files (makes sense), but the problem is that the .ASPX extension is not listed in the IIS Metabase as an application file, so it doesn’t get compressed.  So after enabling the checkbox, you have to do a little bit of XML tweaking to get things working.

If you’re not sure whether your site has HTTP Compression enabled, or you want to confirm that it’s working correctly or what kind of compression you’re getting, there’s a great tool available at:
Pipeboost Real-Time Website Compression Report