I'm still learning, but like already mentioned you need a good clean mix down first. I find that to at least get a loud track after mastering, I actually use light limiting throughout my mix down (especially on the bass and drums). Surgical EQ on all channels is also important to remove any resonant freqs. After this, then I bounce down to an audio file for mastering. This would be around -6dB.
When I 'attempt' to master my tracks, I normally start with a luftikus EQ (free download), and I just lift the 10k region by about 1-2. It gives the track a bit of air. Then I use a saturator (either ableton's own, or camelcrusher, or the softube knob). Then I add a normal EQ just to make sure there aren't any resonant freqs left (so surgical cutting). Then I might modify the mid/side used MSED by Voxengo. After that, a light bit of compression, followed by another EQ where I use very gentle and wide cuts and boosts if required. Then lastly, I add a limiter, and it's normally always A.O.M's invisible limiter.
I say 'attempt' above, as I am not a mastering engineer, and I'm still a newbie. But the above kinda works for me