I recently came across a website that goes into great depth about teaching mix techniques to budding (or experienced!!!) engineers. It’s very thourough and scientific. Possibly the coolest page on the site is the free multitrack download library, where you can find heaps of multitrack audio files (44.1 khz 16 or 24 bit wav’s) to sink your teeth into. There are tons of mixing tricks and secrets here, accompanied with audio files so you can hear what they’re talking about. Here’s the link for the page with the multitracks.