This weekend I wanted to copy some movies that I had ripped with Handbrake to my Apple TV, and to do so, I had to copy the movies to iTunes first from my external hard drive.
No big deal, except that usually when you copy things to iTunes, the files also automatically save to your internal hard drive — a huge pain if you have limited drive space there.
Luckily, I discovered a very simple tip to avoid an unnecessary new file being created and stored. As you're highlighting and dragging the file, just hold the option key as you're dropping into iTunes. The file will be referenced in iTunes and not copied to your actual hard drive.