Say you have a Message Board database, with your main topics and response documents. But you also have a document where you record the Category information. If you have the automatic archive feature set up, it will archive any document whose last modified date is, say, greater than 30 days. Since you usually don't modify the category documents, you don't want these documents to be archived; otherwise, when users went to choose a category for their topic, there would be no choices. To avoid having these category documents archived, run a scheduled agent to change the last modified date automatically. This will keep the documents from being archived.
Create a scheduled agent, called "Update Documents", or something like that.
Set it to run "On schedule daily" and on "All documents in database". Click on Add Search, and choose "By form used" for the condition. Then choose the form that you want updated, in this case, "Category List". This will ensure that only the documents created with that form get updated.
In the actions section, choose "Simple Action", then click Add Action. The action should be "Modify Field", the field name should be "ProtectFromArchive" (if that field isn't already in your document, add it). The value of the field should be 1 and choose the Replace Value radio button.
You're all set now...this agent should run nightly, and you'll notice that the last modifed date on "Category List" documents will always be current.