I'm not to keen on some of the things I'm reading in this thread.
In simple terms:
- Before mastering, turn everything down and turn your speakers or headphones up to the adequate volume to work at. This way you can hear everything clearly but don't rely on compression to do the job, utterly destroying any chance of having a dynamic piece of music.
- Once you're ready for mastering, bring everything up using the faders so that the sum of all of them (on the master channel) peaks at -6db. Make sure nothing in the individual chains is saturating either (for example, effects plugins can saturate too, and cause unwanted issues).
- To master your track and make it fit loudness requirements, apply your EQ, stereo enhancement (if you want it), general polish, and then apply a maximiser (essentially a compressor and limiter combo, but a synergy of both), it will do a better job than a separate compressor and limiter (as one tends to over-react and the other tends to saturate when you don't really know what you're doing).
Now the PEAK volume is completely under control (the one on the master channel that bounces around really quickly), so you need to look at the RMS. This tells you how loud the song SEEMS by taking an average over a certain amount of time. This will tell you how loud your song really is and is essential to the mastering process, along with using your ears.
Consider that certain frequencies appear louder than others to the human ear. This will inform your mixing and mastering on a theoretical level.
Furthermore, a loud mix will use all the available space efficiently. The limiter will make your song quiet if the bass is very loud in comparison with the rest. This is because there's so much energy in the bass, it trips the compressor early.
Balance is everything in mastering.