[Brian]

My workflow doesn't tend to involve a lot of support files, but I think DevonTHINK, Evernote, and Dropbox are the leading methods by which folks manage the files related to their OmniFocus actions.

If nothing else, running some searches here looking for threads that mention them should give you some starting points.

This is one of the more commonly-requested areas to get some attention in future versions of OmniFocus; I don't have any specific info on what our plans might be, but suggestions are always welcome.