I need to use different database connection string and SMTP server address in my ASP.NET application depending on it is run in development or production environment.
<I use a NAnt Build Script to deploy to my different environments. I have it modify my config files via XPath depending on where they're being deployed to, and then it automagically puts them into that environment using Beyond Compare.
Takes a minute or two to setup, but you only need to do it once. Then batch files take over while I go get another cup of coffee. :)
Here's an article I found on it.
The Enterprise Library configuration editor can help you do this. It allows you to create a base config file and then deltas for each environment. You can then merge the base config and the delta to create an environment-specific web.config. Take a look at the information here which takes you through it better than I can.
Have you looked in to web deployment projects?
http://www.microsoft.com/downloads/details.aspx?FamilyId=0AA30AE8-C73B-4BDD-BB1B-FE697256C459&displaylang=en
There is a version for VS2005 as well, if you are not on 2008.
You could also make it a post-build step. Setup a new configuration which is "Deploy" in addition to Debug and Release, and then have the post-build step copy over the correct web.config.
We use automated builds for all of our projects, and with those the build script updates the web.config file to point to the correct location. But that won't help you if you are doing everything from VS.
This is one of the huge benefits of using the machine.config. At my last job, we had development, test and production environments. We could use the machine.config for things like connection strings (to the appropriate, dev/test/prod SQL machine).
This may not be a solution for you if you don't have access to the actual production machine (like, if you were using a hosting company on a shared host).
You can also use the extension "Configuration Transform" works the same as "SlowCheetah",