I disagree with Lavender somewhat. While processes that are considered "mixing" are often part of the composition process, I think the best way to practice mixing (and later, mastering) is to take composition out of the equation. When you're mixing someone else's composition, you'll face problems you probably never even considered when mixing your own work. Different points of view, ya know? She's right that a well-mixed/poorly-mastered track will sound way better than a poorly-mixed/well-mastered track and that you should make sure a track is mixed as best you can before trying to master it, though. Ideally, I think for practice mastering, you should similarly take composition
and mixing out of the equation.
That said, it's probably not going to be easy to find unmixed/unmastered tracks to practice on unless you take a university class on the subject. However, I did find that Nine Inch Nails offers free multitrack downloads, all in .wav format and most in Garageband and/or Ableton Live format, at
http://remix.nin.com. Since some of their final, mastered music is free to download as well, you can also compare your mix to theirs (and the many remixes published on the site). In particular, their latest album, "The Slip," is available for free download at 24-bit 96kHz quality (or 320kbps mp3--whatever), and multitracks for all the songs on the album are available as well.
Also, the Mythbusters proved you can, in fact, polish a turd.