Mastering isn't really rocket science, unless you are trying to rescue a bad recording and mix.
As long as you concentrate on making the mix sound as good and well balanced as you possibly can, it's generally a piece of cake to bring up it's average level to close to contemporary standards (although some of the hottest of the hot levels can be a problem, but who wants to sound THAT loud, anyway?
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Spend most of your time and effort there first, and the mastering will take care of itself.
Don't forget, also, that most MP3 library players (iTunes, etc.) have functions which average out the levels of different MP3's, so you don't need to crush your recording to death to make it loud. The whole 'volume wars' thing that is going on right now was an effort by marketing people in the industry to make sure that their CD's (NOT mp3's) jump out from a pile of other CD's when played quickly by a radio station programmer (louder is, generally, perceived as better, up to a point).
The trouble is, of course, that first, the radio station's broadcast compressors won't let it get loud on air, and your average music listener nowadays takes the CD home, and immediately converts it to MP3's in their iTunes library and rarely ever even PLAYS the damn CD! Whereupon, the iTunes software turns it down a bit to match all the other tracks, and you have lost whatever dubious gain you might have got, crushing the dynamics in the first place
Hopefully, sanity will return to the industry in the light of this trend, and we can go back to music that actually USES a fair bit of standard CD's dynamic range of about 96db, rather than squeezing it down to the paltry few that most modern label releases use...