Some of my music in iTunes won't get Album Art via "Get Album Art". (These might be tracks from physical CDs, or that I've purchased a long time ago from other MP3 stores.) I'd like to use the iTunes Store's Album Art whenever possible, because when it exists, it's usually a better quality image than from those from web searches.
What do I have to do to my tracks to get the iTunes Album Art? What properties of a track does iTunes use when matching Album Art?