The  first principle of mastering is this: “changing anything affects everything” Bob Katz

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A little walkthrough and some ideas to sounds better with your songs.

Make your Song sounds Clear and Boom!
Use a natural DC Offset/Subsonic Resonance removing by adding Highpass/Lowcut Filters in each Channel of your Song. In example: Kickdrums/Basses around 20-60Hz, Mid-Frequencies around 80-250Hz, Upper Mid Frequencies, Highs like Hihats, Cymbals, Rides and every FX you don’t need in the Ground around 300-500Hz. That will save your Ground from a washy and muddy result.

A good way to find the right level of Sub-Basses is to Fade them totally out and listen to the whole song without them and then bring it slowly back into the song untill you can hear it. This way you will always have the right level then.

Make sure that no channel is clipping your DAW(exeptions are selective ideas or FX). Keep it in mind that 2 normal audiosignals can sum to a louder and clipping result. In the most modern DAWs(like Ableton, Reaper, Cubase, Logic) you can select a group of faders in your DAW mixers and draw them down as a group till nothing is clipping. The reason of this is : You will save the mix situation between the channels and instruments when you move all important faders as a group.

Using Lowpass/HighCut Filters and Shelving Filters
Do you want this sounding warm like an old vinyl and kool song that remind you of your youth? Then you should use shelving filters over 10-12Khz to reduce the highs a bit and sound warm. The Reason for this is that an old hardware was not the best tools to sound high fidelity and after each processing or recording like Tape you will lost a bit high frequencies. Try it out with different settings according to your songs and play with that idea.

Monitoring/Leveling
I think this is a very important issue and frequently misunderstood point to mix your song on a lower level and only go up with the monitor level from time to time, not with the master or channel faders in your DAW. Its important to improve quality while mixing your song. (similar to the way you should mix the subbasses into your song – see above)

Keep in mind
Digital waveforms are only 1 and 0 this means that a full 0dBFs Wav/Aiff-file with 16bit is totally filled with 1. Every DA converter(MP3 Player, CD Player, Audio Interface etc.) needs headroom to sounding good and this is the reason why you should put your Audiofile on -0,3dBFS to ensure a good quality bounced Audiofile. You can also add a brickwall limiter as the latest plugin in your master channel to avoid clipping peaks and leveling to -0,3dBFS.

All digital waveforms can only be a copy of a natural and/or analogue sinewave. Your AD/DA converters for example your( Audio Interface) will build this digital copy with square steps and aliasing bridges between them.

Dithering is very important to improve a good aliasing between these steps and to make sure youre song sounds good on a lower bitrate/wordlength. But only as the latest step in your Mixdown, when you reduce the processing wordlength/bitrate of your DAW(32bit float or 64bit foat) to a final Audiofile in example to 16Bit 44Khz

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How to avoid over-Compressing your Songs using the TT Dynamic Range Meter
(I recommend you to go not higher than DR 15 for preparing your files)

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Mastering Guru Bob Katz about
Level Practices 1
Level Practices 2
Mixing Tips & Tricks

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Robert Henke aka Monolake about
Mastering Audio
How you produce an Album without using a Compressor

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Chris McCormack about
The Loudness War