I know none of you know me, but honestly, there is a lot of bad advice in this thread already.
Mastering is NOT a sacrifice of detail for loudness. If it is, you're mastering wrong.
A VERY BIG PART of mastering is the ability to separate your mindset from the mixing stage. It is a completely different ballpark. Mastering is about optimizing a song for playback across a wide range of sound systems. This CAN be accomplished with mixing, but not everyone knows how to mix and a mastering engineer has the job of fixing playback issues that may arise. Mixing is about optimizing individual elements of a song to be heard across multiple sound systems. The goal of a mastering engineer is to do as LITTLE AS POSSIBLE to the track as to not change the artistic presentation of the song. Integrity is EVERYTHING. Avoid using multiband compressors unless the song specifically calls for it. Avoid using harmonic excitation if the track doesn't need it. Being an ME requires a VERY critical ear and you have to understand when you are causing problems with a track and when you are helping a track. TOO MUCH IS TOO MUCH. Barely anything can often do the trick. And again, fixing it "in the mix" is often a better solution. One thing to try to accomplish is HARMONIC BALANCING in your mix. Make sure everything is mixing to a generally flat eq curve, but have the bass hit a bit higher on the eq than everything else... maybe 3-5db louder than anything else. Sometimes you want your lead sounds to be the same volume as the bass. A good way to check your EQ curve is to get a spectrometer thing a ma bob like Stillwell Audio's Schope First thing to do as a ME is to balance the EQ harmonically so all elements of the song can be heard equally, or well, or however you want.
One big thing to understand is perceived volume vs actual volume. Which is basically the battle between RMS and PEAK volume levels. Compression plays a big role in this. For mastering you DON'T WANT TO USE A LIMITER TO ACHIEVE PERCEIVED VOLUME. Limiters are for catching peaks and not squashing a mix for perceived volume. You will RUIN your track by doing this. What is good to learn though, is how to properly use a compressor to reduce your dynamic range. A good mastering compressor will be TRANSPARENT, you don't want to hear it working. Mastering compressors usually go for a pretty penny in the hardware arena, but you can get a nice (and new!) digital mastering compressor from Stillwell Audio called Bombadier. You want to affect the RMS of the track and raise it to an acceptable level. One of the most important parts of this is to know how Metering works. You want your master volume meter to stay at a high level while peaks and valleys are reduced. I personally like using compression to reduce my dynamic range to 5db or so, with peaks hitting around 0 to -1db and bass and stuff riding around -4 to -6 db. But realize this, dynamic range is dependent on the program material. If you want more space and whatnot, you need to have a higher dynamic range, if you want more of a wall of sound thing going on, you need a lower dynamic range. I only use a limiter to catch errant (often random) peaks that you do not hear in the music but are happening in the waveform. My limiting will rarely kick at more than -4db of gain reduction. And that will be but for an INSTANT.
EQ is a VERY powerful tool in the mastering stage and should not be neglected unless the song itself has no issues. Most of the time, you will be doing subtractive EQ to fix problems. Sometimes, but not not often, additive (better hope you have a VERY nice EQ for this). Most EQ changes will be no more than -3db. If there are, then something needs to be fixed in the mix, if thats not possible (say the mixing engineer is of town or something), you have to do what you can. Again, better hope you have a NICE EQ. Linear phase EQs are great, but sometimes Minimum Phase EQs are nice too (specifically on high frequencies, linear phase is better for Bass frequencies) Try this: 12db Highpass filter set at 30hz. This will often clear up rumble issues and bass that overpowers your mix. Cut 3db at 18khz and 11khz to make things smoother. Sibilance issues can be cleared up in the 3-11khz range.
M/S processing CAN be done in the mixing process and more often than not, its a better idea than having the ME go ahead and try to create more Side gain out of what is there. This can lead to phasing issues and its only necessary to add M/S processing if the track is lacking "width" or "space". You can often create a bigger sounding track with M/S processing, but if its done in the mix, you don't have to do it in mastering. (I personally do a lot of M/S with my lead synths and upper freqs of my basslines)... try M/S in your mix, you can come up with neat effects and if you know what you are doing M/S also has the ability to create perceived front to back placement. With delays and M/S you caqn also create sounds that move forward and back (around? ). I will sometimes reinforce the Mid and Side if a track needs it. Most of the time Side needs some addition. (But with the way I mix my music OWN music, I rarely need to mess with M/S in mastering)
The best way I have found to approach mastering endeavors is to take it one song at a time, learn how to make one song sound good on multiple sound systems (reference tracks ftw). Once you can do this, it opens the doors to mastering other material.
I can go on about each of these subjects, but honestly, everything is program dependent. Whatever the song you're working with needs, it needs. If it doesn't, DON'T TOUCH IT!
If someone would be so kind to offer a track for me to master, I can go ahead and do it, and then tell you guys the steps I went through to optimize it for playback.
(and if anyone doesn't trust my statements, I've had my music played over a $20k sound system and been told it sounded amazing, not gloating, just stating a fact)
A good resource for mastering information is http://www.gearslutz.com. They have their own dedicated mastering forum where such names as Bob Katz and Robert Babicz often stop by to help out. (If you don't know those names, you BEST be googling them right now!)
P.S. I really don't like Ozone. Its EQ sounds terrible, its harmonic excitation is only applicable in some cases, its limiter sounds terrible, and the rest of that stuff I don't use. The only thing I like is Ozones masterful approach to controlling the stereo spectrum. I have used it enough to know I don't like it and have my own specific way of mastering that utilizes different plug-ins and different techniques. I DO NOT USE MULTIBAND COMPRESSORS really ever...