I'm working on an ASP.NET app that uses VSS for source control. WE have a PROD server and TEST server. As things get built, they're published to the TEST server for user testing. Once accepted, they're moved to the PROD server.
I need to work on some new functionality in a sandbox without affecting the existing build in TEST. I don't want what I'm working on to get mixed in with the code base that's TEST code already published because I need to be able to make fixes to TEST code without worrying about what I'm doing in the sandbox.
How do you keep the TEST & sandbox environments exclusive?
What you need is what's known in source control systems as a branch. Here's a guide on working with branches in VSS.
Basically you'd branch off your TEST version, do your sandbox feature and then if you ever want to include it in the main TEST version merge the sandbox branch back.
This image nicely illustrates the idea:
Making a branch should help. See your source control documentation about that. If you still didn't choose one, I'd recommend git, for it has good specialization in that aspect (though a little higher learning curve).