There are lots of good articles online about various mastering techniques. The real important things you need to know about are EQ and Compression. An important rule of thumb is to have good reference material to compare your track to. Always listen to one of your favorite songs that is a similar genre of the song you are mastering first. Go back to it at any time to refresh your ears. The human ear and brain have a strange capacity to compress and equalize by themselves so you need a common frame of reference. There are no rules that are necessarily set in stone, but there are some guidelines that seem to work most of the time. The goal in mastering is to get things to sound reasonably natural in as many different listening situations as possible. The idea is not necessarily to get things as loud as possible, but it seems that a lot of people think that that is important. A good place to start is on EQing the roll-off points on the high and low end. On the low end, rolling off somewhere between 30hz and 60hz seems to allow enough room for you to get your mixes up to a reasonable loudness level. The High mids starting from about 1khz all the way up to 20khz seem to sound best when they smoothly roll off, roughly following the fletcher-munson curve for 85db SPL. For CD and/or internet release, things can be prevented from sounding too tinny on some devices if you have a good round curve starting from 16khz going all the way down to nothing. All of this is a rough guide and is purely subjective. Again, on the low end.. you really just have to use your ears and move the shelf around a bit until it sounds right.. but usually its somewhere between 30hz and 60hz depending on how much energy you have down there. The more you roll off the low end, the louder you will be able to get your track to sound, as low frequencies eat up more energy. Tooling around on the extreme highs is the same kind of thing.. just screw around with your high shelf until it sounds right.   For the mids, well. the idea is to get each instrument to sound clear but not unnaturally isolated. Also. if your track sounds muddy. you can usually remove some mud somewhere between 400-800hz. Most of the time you can deal with very small db increments on a parametric EQ that has its q set to about one full octave. .which is 0.30 on most eqs. Ill get back to you on the compression/limiting side of things. Need to go work on my track Hope this helps some.

On the loudness tip..  When mastering, I personally shoot for about -12db RMS. That makes it close enough to commercially loud, you can listen to it in a car with no problem, but still allows a fair amount of dynamic range. For most of the zeros, this was pretty much the normal commercial level. Now it seems that people are shooting for -9db RMS and above.. which is ok if you are doing aggressive dubstep/tech house (skrillex's album clocks in at about -7RMS for the whole album), but completely unnecessary.