When you hear your tracks repeatedly, over-familiarity pushes you to ignore the mistakes. You need a fresh perspective and therefore you need a third-person, preferably an expert to master your tracks. Someone who is hearing the music for the first time has a better chance of recognizing the subtlest aspects and fixing the mistakes. Ever had someone proof-read a paper for school? Then you know how helpful a fresh set of eyes (or ears) can be. No author is allowed to edit their own book for publication, and we say no musician should be mastering their own music!

Careless handling of the midrange EQ during audio mastering can quite easily be a detriment to the sound. Too often, EQing while mastering is focused upon the extremes and there is very subtle dip in the mids. This approach is fine but must not be too frequent or too exaggerated. The mid-range of a song must be heavy enough to bring out the richness of the harmonics. 

Talk to your mastering engineer about the compressors used for mixing. Multi-band compressors are the ideal choice as they make the entire process flexible, easy and transparent. The multiple bands of the compressor must be treated with different settings to ensure that there are mild shifts in frequency and the different contours of the musical arrangement are justified. Music mastering services with multi-band compressors are preferable as this avoids the use of more complex techniques like automating a static EQ.

While amateurs focus primarily on turning the track into a commercially attractive one, there is just more to it than that. The mastering process is aimed at giving the track a final check and detecting all technical flaws. From balancing the track from left to right to centering all elements appropriately, mastering is your final chance to look over the track and set it right. If you are opting for a reliable online mastering service, then you are in safe hands with Sola. 

When you listen to mastered songs, each track will vary from one another drastically and yet listening to them one after the other will offer a good experience. For instance, one track may be too thick and smooth while another one may be thin but punchy. It is the duty of the mastering engineer to ensure that despite the tonal differences all songs fall within a range, minimizing discordance while maintaing a pleasant variety. Keeping a set of reference tracks can help the musician greatly.