If loudness is what you're going for you have to mix your track with the intention of making it loud. What I tend to do when mastering for loudness is having something like the Ozone EQ to monitor what my RMS level and Peak levels are doing. Then it's your standard EQ and compression but I also clip certain elements that have a lot of transients, like drums, and bring down the peak level. I also tend to sidechain compress stuff aggressively or use something like LFO tool/volume automation to drop it to inf. Then when mastering I do my standard EQ, Mid/Side, Stereo Width but I also soft clip the track using Logics bitcrusher, Kazrog makes KClip which will do the same thing for both PC and Mac. I monitor my Peak/RMS and bring them as close as I can and bring the level back up with the AOM Invisible Limiter or whatever limiter you prefer. Many will tell you that clipping is bad and you should avoid it, and if you don't understand what you're doing then yes you should avoid it. But nowadays when mixing in the box clipping can be a very helpful tool. Hell even highly respected audio engineers will clip certain elements.

I had heard about this but whenever I tried it out it I never got justifiable results. However, saw this today (although he doesn't use a send in this video) and for some reason it all just clicked in my head.
https://www.youtube.com/watch?v=ZDQSnV0cBMg

Mixing in mono is usually a reference technique to make sure the song sounds good for playback on Mono systems like some clubs, radio/broadcasting stations, stores, etc. Some people swear by it and will mix the majority of a project in mono while others rarely think to check it. I personally think it's good to check it every so often to make sure you're song comes out sounding ok in mono, but if you're only releasing things on say soundcloud and you don't have a huge following I wouldn't worry too much about it.

I (at least when mixing electronic music) mix as I go. Separating the mix stems from when everything was recorded on to tape and then mixed on a console (but even back then people would EQ and compress .etc while tracking). But when working within a computer, with VSTs and such, it makes sense to just mix as you go along simply because you can. But to each his own.

I remember hearing about this plugin but didn't really look into it. This looks really fun