You can learn as many things as you want about mixing, but you have to get your hands dirty by practicing because that’s when you understand if a technique fits your style and on what scale you should apply it.
Your will to get better isn’t enough though, you need the actual raw material to work on, which are multitracks.
Here’s a list of websites where you can find multitracks:
The Pro Audio Files over 200 hours of mix tutorials and over 25 sets of multitracks.
Pro Mix Academy All tutorials include downloadable multitracks.
Mixthru Start to finish mixing tutorials with free multitracks included.
Tourna-Mix 2 Monthly mixing contests with quality multitracks.
The Mix Academy One multitrack per monththat you can use to build your portfolio.
Nail The Mix Get two multitracks when you sign up, and then one each month.
Modern Mixing Library over 20 premium multitracks that you can purchase and download on demand.
PureMix You get multitracks from past and current mixing contests (currently 40+ multitracks), as well as multitracks from some of Fab Dupont's tutorials.
Shaking Through over 50 fantastic indie-rock multitracks (along with appreciable bonuses such as recording notes, exclusive videos, and more).
Ken Lewis’ Audio School Online Multitracks are included with many of Ken Lewis' tutorials.
Telefunken Free multi-tracks recorded with Telefunken microphones.
Cambridge MT Huge archive of free multitracks for training your mixing skills.
Nine Inch Nails Awesome collection of multitracks for remixing.
Shaking Through Sign up for a free account to download the multitrack from their latest episode.
PureMix also offers some free multitracks:
Lewitt Audio Microphone manufacturer that occasionally makes mixing contests. The multitracks of past contests are available for free.
Now go and improve at your craft with all these awesome sources!