If I could have my way all the time (shya!), I would never master my own music. Why? Because there are people out there who do NOTHING but audio mastering all day every day. It's a highly specialized skill, and the people who do it professionally have super developed hearing and, usually, high-end gear designed for the purpose.
However, I understand that I will NOT have my way all the time. And that means I won't always have as much money as I would like (well, who does?:)). So it is sometimes necessary to do your own mastering. For times like those, it is best to have at least SOME knowledge of what you're doing and why you're doing it.
For an overview of what mastering is, you can check out my article Mastering a Song – What Does It Mean?
Here is a handy article with twenty additional tips for mastering your own audio: