From what I've gathered thus far, mix theory is not something one learns from one lesson, or from an entire course for that matter! Practical application of mix theory is something that only experience - trial and error can teach. Of course, however, lessons and a mentor will greatly accelerate this process. I shall list some key elements that make a mix great.
Balance - If mixing had to be summed up in one word, this would be the word.
Elements to control which are tools for BOTH balance and creativity are: Levels Panning Equalization Dynamics Processing Effects Processing Automation
Some common (and ever vital!) general tips are:
Back down the levels of your effect sends, since they will be magnified in the mastering process
Check your mix in mono, this will reveal problem stereo tracks/effects and phase issues (your track will not always be played back in stereo!)
Bounce down to the right format (and sample rate/ bit depth)
Take Breaks! I've found this to be one of the most important tricks in mixing and I almost feel it should be be taught as theory. It's amazing how quickly our ears lose perspective, and we end up tweaking things that shouldn't be tweaked and we end up working against ourselves and our ultimate goal - balance. To minimize this, simply take a 5-15 minute break after every hour (at the most) of mixing.