Recently I've been defining my downloading ethics and this is what I've come up with. I've realised that for me it's mostly about whether or not I am getting something for free that I could not have gotten for free by some other means AND depriving someone of their rightful income in the process.
1. If it was shown on free-to-air TV, I can download it. That's just convenient scheduling.
2. Generally I don't download movies (any more) but I make an exception if it's a Japanese movie that I can't easily get hold of, and particularly for fansubbed versions of movies that didn't come with English subtitles on the DVD, because if I couldn't have understood it, chances are I wouldn't have bought it.
3. I can download any album/audiofile I want to hear, because that's no different to me standing in a music store listening to the song, but if I like it enough to listen to it more than twice, I owe the artist their income and should buy it. And I mostly do. The area where I really fall down here is in comedy. At some point I really need to buy Eddy Izzard and Jeff Foxworthy's complete CD/DVD collections, because I own not a one and I have listened to all of them at least three times each, so far. Good stand-up comedy just plain makes my life happier.
4. I can download any song no longer available for sale, because I can't get it any other way and the artist didn't lose a sale. This comes up with indies bands mostly. Their early stuff is usually released in limited numbers and at some point becomes difficult or impossible to buy.
It's been said that file sharing doesn't significantly decrease sales (the estimate is around 5%), because in most cases people would rather forego the music they download than pay for it. They listen to it only because they can do so for free. That makes sense to me, but on a personal level it's irrelevent. At a certain point I always know whether I like something enough that it has become something I should buy.
I'm comfortable with those rules of downloading and I do stick to them. The one thing I'm not sure about yet is the fact that I share files knowing full well that I have no control over how the people downloading them use them. I can say they are shared for evaluation purposes, ask them to support the artist by buying the CD if they like the music, and tell myself that the ethics of the matter are up to them after that point. But I haven't quite decided if actually true or just a way of excusing myself. Anyway, downloading shared files without sharing is scummy, so as long as I'm downloading anything, I'll always be sharing something as well, even if it's just in the form of leaving that same file in the system for a while after it downloads (as with BitTorrent).