One of the best ways to understand mastering is to listen to a lot of commercially produced music, but do so with a focus on really paying attention to the sonics as well as just the music. You’ll find that it changes a lot by era as well as by genre. So, pop music from the 1980s isn’t usually mastered all that hot, but rock from the 2000s is often really crushed in terms of its dynamic range. Narrowing your focus down to the kind of music you are making, literally A/B’ing tracks with your own is a great learning process. How much stereo width does the commercial track have? How much dynamic range? If yours is different, in what ways is it different and how can you make it more like the mastered track? If mastering for vinyl, be sure to compare with other vinyl and not digital tracks.
This is related to the previous tip. As well as listening to tracks to compare their treatments, you can analyse them. Listening is great but it does put you at the mercy of your physical space and monitoring setup. Digitise a track and load it into a wave editor, or call up a spectral or other type of analyser and you’ll see the track’s characteristics displayed graphically. Where is its energy, how much top and bottom end does it have? This is an invaluable way to see the “shape” of the character of a production.
Your DAW will come with some compressors, EQs and limiters that can be useful as part of a mastering chain. But there’s other free stuff out there to download too. To name just a few: Voxengo’s Marvel EQ and SPAN meter, LoudMax Limiter, Stereo Tool by Flux and mvMeter by TBProAudio will all give you better control over and insight into your masters. There are many others around as well, should you care to grab them.
Until you are very experienced at mastering you’re unlikely to get it exactly right on the first attempt. The way to judge how your master translates to different playback systems is literally to play it back on them. Luckily this is essentially free - take your track and play it on as many devices as you can get access to, noting the way it behaves on each. Too much bass in the car? Not enough mid on earbuds? These are all things you can go back and iron out, even if it takes several tries.
This is a very subjective thing, but unless you are extremely confident in your abilities, it’s always good to get a second opinion from someone whose ears you trust. If you’ve written, recorded, mixed and mastered a track you’re likely to be almost over-familiar with it. A fresh pair of ears can really help to being new perspective. They may not necessarily say anything groundbreaking, and you may not agree with them if they do. But this process can sometimes help to make a good master into a great one.