I'm a bit wary of the proposition, as I think mixing is kind of a game of inches, and no one particular thing provides a huge difference on its own.
 
However, I'll throw in this: once I decided never to spend more than a day on a mix, the rate at which I got better went through the roof. Coming back and making refinements on a recall is fine. But if I'm generally not happy with a mix, I never tweak what I've got, I flatten it and start again.

1. Use a High Pass Filter on almost every track to clean up the sound but also to free up headroom.  Example: Acoustic guitar thumps and rumbles below 100-150 HZ just take up space.
2. Make sure the bass and kick complement each other.  Example 1: You might cut the kick around 300 hz to eliminate muddiness, but either boost the bass there or just leave it alone so it is defined in that range. (All depends on the song and instruments.)  Example 2: Decide which of those sounds is going to represent your low-end power and get the other one out of the way.  You don't want a lot of 40-80 hz on both sounds.
 
You can see from this that I consider getting the low end right the most problematic area. 

Building effective sound absorption, ARC, thus reference material and my mixes more accurately. A very close #2 is CUT - DON'T BOOST! There's a large number of small things that contribute that is growing steadily in my world.

konradh
You can see from this that I consider getting the low end right the most problematic area. 

Amen brother. I still have yet to solve this, but I have a feeling that 12 stylus loops stacked on top of eachother plus a monster trilian bass patch aren't helping my bottom end. Lol

If I could rewatch that fabfilter vid on the exact carve/space concept everyday I would. And should!!

Learning how to gain stage properly
Cutting instead of boosting
Room treatment (partial)
Listening to mixes on as many systems as possible
Not being afraid  nuke entire clips

caminitic

konradh
You can see from this that I consider getting the low end right the most problematic area. 

Amen brother. I still have yet to solve this, but I have a feeling that 12 stylus loops stacked on top of eachother plus a monster trilian bass patch aren't helping my bottom end. Lol

If I could rewatch that fabfilter vid on the exact carve/space concept everyday I would. And should!!

I find you have to carve a ton out of the low end of Trilian patches. 

konradh
1. Use a High Pass Filter on almost every track to clean up the sound but also to free up headroom.  Example: Acoustic guitar thumps and rumbles below 100-150 HZ just take up space.
2. Make sure the bass and kick complement each other.  Example 1: You might cut the kick around 300 hz to eliminate muddiness, but either boost the bass there or just leave it alone so it is defined in that range. (All depends on the song and instruments.)  Example 2: Decide which of those sounds is going to represent your low-end power and get the other one out of the way.  You don't want a lot of 40-80 hz on both sounds.
 
You can see from this that I consider getting the low end right the most problematic area. 

I recently stopped high passing the crap out of everything and I think it sounds better. What I do instead is give things a moderate HPF cut around 80Hz or so (higher depending on the frequency range of the instrument), and reduce the low frequencies where necessary with a low shelf. I think if you HPF too much you lose a lot of the warmth in the mix and things end up sounding too brittle. Whereas if you use a shelf to attenuate the lows instead of getting rid of them completely, you stop them muddying up the mix whilst retaining some of the warmth. 

savoy

ce
cut instead of boost

yes mister!

I think this depends on the style of music. When people advise you to cut instead of boost, they're talking about the fact that boosts can sound unnatural. In some styles of music, especially contemporary and electronic styles, this might be just fine because you're not exactly aiming to make things sound natural. The artifacts of a boost might just be what you want. I'm always surprised when I see producers like Dave Pensado work with EQ, they sometimes use huge boosts as well as low-Q cuts (we're always told to cut narrow and boost wide). But I guess that according to their professional hit-producing ears, those boosts worked. 
 
A similar thing applies to the old "use reverb to make everything sound like it's in the same space" advice. Again this is very helpful if you're trying to make a mix sound like a band playing in a room, but sometimes that's not what you want and a mashup of different reverbs gives you the sound you want. 

I've learned SO SO much from this group over the years. I think one of the single biggest jumps in my mixes came from learning how to gain stage - in particular how to use VU style meters to gain stage. There is a great thread that really made a difference for me...

Don't waste 4 hours trying to use all the "nifty tools and gadgets" we have to correct something that could as easily be done by 10 minutes of re-tracking.