I'll start with the hard core facts.
If you are serious, forget about doing it yourself and get a RO ME to do it for you.
If you don't have great monitors and a great room, you are already at a mass disadvantage and will be going in circles. I read over and over how people are buying this and that thinking it will improve their sound or mixes while they miss the big picture.
It truly is all about your ability to hear the sound properly in an acoustically treated and tuned room and then, have golden ears and stellar gear and the knowledge on mastering music.
Also, if you are mastering in the same room as you mixed in, you are only compounding the problems you aren't aware of. You need a second system in a different room to really help. It's also advised not to master the music you mix but we are all on a budget these days and are doing what we can to get buy.
If you are mastering your music, before you spend a dime on plug-ins, treat your room for certain.
Looking forward to other comments