I never EQ the master bus, recently my master bus usually looks like this:

ozone maximizer (loudness)>oxford inflater with the clipping off (for filling in harmonics very slightly)>camel phat with everything off (to bring it back to 0)>oxford limiter (for a bit of extra loudness + slightly filling in harmonics again and limiting obviously)

I'm a bit surprised to see that people are using so many plugins on their master channels. Is there a specific reason you apply more effects to the master instead of doing it inside the mix somewhere or does it not matter?

How does this "everything under 150Hz has to be mono" works, I mean, how do you turn something that has a very wide stereo image into mono?, the kick and the bass for example; how do you know those low frequencies are in mono? and how do you make them mono if they are not?, and what are the benefits for having them that way.

Maybe I'm asking a lot of things but this particular issue is having me troubled and intrigued.

To turn a sound completely into mono, just turn your mixer track into a mono track. In FL it works like this (stereo knob turned all the way to the right = mono): http://imgur.com/iWuAjsz
If you're not using FL, you'll have to consult your program's manual to find out how to do it there.

To only turn the lowend of a certain sound to mono, you can use Pro-Q2's mid/side EQ like this: http://imgur.com/II8FOxi
What it does is it cuts away the side signal of the sound with a "low cut" filter. In very basic terms, the mid signal is whats equal in both the left and right channels (mono), and the side signal is what's different in both channels (stereo).

In my opinion, you shouldn't split sounds too much as it can often mess with the dynamics, so I can't recommend using too many multiband plugins. Instead, try to think about what needs to be wide and what doesnt, keep it simple. In most cases it's enough to just make the whole sound either mono or stereo. And remember, it's not the overall width that makes something sound wide, it's the contrast between mid and side. My rule of thumb is to keep everything that has to sound precise (low basses region and drums, lead vocals) more mono, I leave the stereo space for sounds that require it to work (pads, chords or sometimes background vocals).

What helped me heaps was a technique I call "band mixing".

To help you with it, you should download a free visual analyzer called SPAN which displays your frequency response on an X/Y graph. While the way your mix looks isn't as important as how it sounds, a visualizer can help you diagnose potential issues in your mixdown quicker.
http://www.voxengo.com/productversioncheck/span/?version=2.4.1

I load up SPAN and an EQ plugin, then put my own track into my DAW next to another one, both going into the same mixer bus. I listen to the lows, mids, highs seperately and compare them to the other track (A/B) and try to find out the differences between what I am doing and what my reference track has going on. This can be a good way to develop a better ear and better understanding for what your mix should sound like at the end.

Another way to do it is to put a lowpass in at around 100 hz and to keep sweeping it up in small steps while A/Bing until you start noticing a difference in quality between your track and the reference track. You might notice weird peaks in your song (either audibly or visually in SPAN) that aren't there in the other one in the process, allowing you to diagnose potential problems. Also, sometimes when I'm clueless about why my track isn't flowing well, I'd do this only to find out everything was fine apart from the crashes and hihats.

Starting from the lowend and working your way up makes sense because the bass regions generally dont have as much going on as the mids and highs. As a result it becomes easier to compare them, and as you gradually move your lowpass up you can start focusing on each region of the spectrum on its own. Just remember that the level of one region affects your perception of another, so try not to focus on all the regions on their own too much. Instead make your changes in context with the rest of the mix. Most mix problems you can't figure out stem from the mids or highs and their dynamics, plus their relationship to each other and with the lowend.

Finally, keep in mind that every song is different, your record might call for a different mix than the reference track does. This technique can just help you analyze other mixes and take them apart to help you study good songs, but you'll still have to spend hours on your mixes to understand what really makes your music shine.

-v