Creating scripts to administer ISA Server 2004 doesn't have to be rocket science because the ISA Server 2004 team has provided an excellent reference in the ISA Server SDK. The best way to improve your skills is to read through the SDK and practice creating scripts in a test environment. Keep in mind that it might take some time to become familiar with this SDK. To see some excellent sample scripts from the ISA Server 2004 team look in the S\Samples\Admin directory on the ISA Server 2004 CD.
You can also find some excellent scripts on the Internet, which you can use as a sample to create your own scripts. To get inspired see http://www.isatools.org, where Jim Harrison frequently posts new scripts.
For more information on these scripts, see the "Additional Resources" appendix, and http://www.isamvp.org/apc.
For more information on how to script with ISA Server, see http://www.isascripts.org.