You're going to need some sort of tool to look at your storage and your different file systems, and to help you make those decisions. The tool should tell you what the file activity is, including how many files you have, when they were last accessed, the size of each file and who used the file. Once you can quantify these types of issues, you can make a more informed choice about the file's relative importance. Based on that evaluation,...
you can decide which file(s) to archive. Basic search or storage resource management (SRM) tools can do this on a high level -- some tools can even examine deeper metadata to evaluate additional file attributes. Then there are deeper, content- or context-sensitive tools to search and discover files. Some of the archiving vendors provide tools to help you identify candidate files and analysis or modeling tools to help gauge your benefit of implementing archiving, for example, to perform what if analysis of the impact of migrating files meeting a criteria or to test policy rules and implementation templates. Some storage system and in particular some NAS vendors are beginning to provide integrated archiving and file migration as part of their core solution offerings in addition to host server or appliance based solutions.
But any file archiving decisions will also depend on your business requirements. It's one thing to identify a candidate for archiving, but the issue of "should it be moved or archived" is an entirely different consideration. For example, it's easy to just delete all MP3 or WAV files thinking that they're only music, but some of those files may be part of an important presentation. It's not enough to simply classify a category of files. You need to know that each file can and should be moved.
Go back to the beginning of the File Archiving FAQ Guide.