Hello all...
 
I can't tell you how much I've learned about music composition, production, and mixing just from reading the threads on this forum.  Beyond amazing.  Along the way I've posted about guitar sims, mid/side stuff, EQ notching, and a million other things I've yet to decode when it comes to the art of mixing.  Sonar user since 1998, which is why I'm here instead of Gearslutz...  =)
 
I wanted to start a thread, as the title says, on the best A-HA!! moment that took your mixes to a new level.
 
Too many for me to list, but parallel compression was a giant one for me...so was mid/side EQing (via Fabfilter vid on YouTube)...thanks to whoever recommended that.
 
So...fire away!!!!

I think for me it was coming to terms with the fact that not everything has to stand out clearly in a mix, that a good mix has a healthy contrast between dark/bright sounds and front/back sounds. If everything sounds bright, then nothing does. Sometimes you just have to roll off the highs on a track and have it sit in the background.

Also don't be afraid of panning things 100%, it creates width and clears the middle for the important elements of the track (kick, bass, vocal etc). I used to pan everything from left to right in a rainbow thinking that I had to use up every "space," but it just ended up spounding cluttered. Now I take an LCR approach, limit the number of stereo instruments to one or two, use stereo reverbs sparingly (and narrow their width if necessary), and if I pan things outside of LCR I'll do that sparingly too (perhaps a subtle mono reverb or delay).

Also: you don't have to add distortion to everything. Too much in a mix can make everything sound brittle and fatiguing.

And: don't be afraid to make major cuts to get the bass sitting right in the mix.

This is going to sound really stupid but learning what the heck a bus is and how to use them. I recorded and mixed an entire album for my old band on my old system/DAW without a single bus except for the Master (meaning I did not add or route to any busses at all).
 
Imagine my surprise upon realizing I could keep all the noise from the drums, vox and my umpteen guitar tracks going to their own little homes within the mix.
 
D'oh!

cut instead of boost

i can break down the many things (a lot from coming from this forum) that have improved my mixes into the following:
 
1) Focusing on learning a few plugins well instead of grabbing for a new plugin every mix. At this point  Quadcurve EQ does the bulk of my EQ work with the occasional appearance of a Pultec or other coloring EQ. 
 
2) Following the advice of forum members in regards to gain staging has been a true eye opener.  It seems obvious now, but like many of you, i grew up with analog that required a hotter signal.
 
3) Echoing sharke, the attempt to make every element sound perfect not only kept me from finishing mixes but it was actually muddying my mixes.  If you have two elements that sound perfect in a vacum on their own and they share a lot of the same frequencies, odds are good they are going to ruin each other in a full mix. Grasping that getting elements to fit well together beats a bunch of things that sound good soloed was a challenge for a bit.  Now the only time i hit the solo button is for forensic tasks like removing noise or whe nfocusing on the one element i decide is the key (usually the lead vocal)

In the beginning it was modest room treatment, ARC and a sub.
 
Then it was simply listening carefully to what I heard in the speakers, cans (4 different types) and the car.
 
Lastly, being open to what others hear. There are some members here whom I trust to listen to pre-releases; even if I do not always take their input 100% to heart their input is IMPORTANT to me. And, whatever else I may get from posting in the songs forum.

Learning that every instrument doesn't get to utilize the full frequency range it produces and the fundamental frequency isn't always the one you want to preserve.

The track Mute button.
 
Both a joke, and a serious point..

technique - mid side EQ
plugin - BFD3
fixed mistake - getting rid of too much 100 to 300 hz in mixes (when I discovered room treatment and purchased proper near field monitors)

Analog front end.  It really makes a difference for me.
 
The other thing is learning to listen.  It really does take time, but once you reach a certain stage suddenly tracking (and mixing) start to make sense and you can learn to apply the right tools rather than just twisting knobs and hoping for the best.