I check 3 main things with mastering.
1.) Frequency Range: Although it may seem like there isn't much frequency activity below 30hz or higher than 20khz, I always make sure to cutoff this range with a high pass/low pass filter. Not really a big deal, but it does clean up the sound. The goal of mastering is to pretty much level out and fill this range with as much sound info. as possible. That being said, how you approach this is really grey area.
2.) Volume: Make sure your most prominent (for trap sounds [i.e bass/kick, snare, hats]) sounds that actually carry and progress the track are the loudest. I usually use a couple different 808s, and sidechain them with kicks depinding on how bassy (aggressive/dark) the track is. Just make sure that when you're composition is in place and the track is structured pretty solidly to measure your levels (also meaning have every sound tracked to a mixer!!). I usually run snares at -6db and have the hats a little lower. As for the bass.. it really depends on what the artist or target market you're looking for. I try to keep kicks a little louder than the bass, but still maintain a solid level (louder than samples/pads/leads).
3.) Mono/Stereo sound: I also isolate the mono sounds and check those vs. the ones that are stereo separated (like left and right surround). This can be achieved through reverb and stereo separation plugins but basically it's letting each sound be heard more clearly by giving them a little spot in the song's environment. Sound IS three dimensional; you can hear things above, below, to the right and left, and also by adding different reverb tail lengths you can give a song major depth perception within your sounds.
Hopefully this helps, and as a tip for the master track I run a couple EQ's first (one tight and surgical to reduce "ringing," another more smooth for boosts/cuts), a compressor for slight gluing and bringing up softer sounds, and a limiter when it's all said and done. My biggest rookie mistake was mixing with the limiter already on the master so everything was distorted yet at "0db" so you couldn't really tell that it was not as expressive as it could've been.
- Pool
