Hi - As much as I agree with the Bear that Mastering is best done professionally, I'm a little less convinced that it can't be done by the home recordist, or that any attempt to do so will end up butchering you material. But it is something that is very different from mixing, and can only be successfully approached with a completely different mindset than the one you mix with.
Mixing is a creative process, mastering is a surgical one. Everything you do in mastering is for a specific reason. Turning knobs to see what happens doesn't cut it. It's too easy to fix one thing and not realize that you've completely whacked everything else in the process. And with cheap monitors in an untreated room, you'll really have a hard time.
If you approach mastering the same way you approach mixing, you'll just be trying to accomplish the same thing you we're trying to do during mixing, only with less control because you now are working with just a stereo track. For this reason, going into mastering with a mix you are not happy with is doomed from the start. Read this paragraph again!!!
Once you have a mix that you are happy with, then there's some things that the home mastering engineer can do, and do well. The first step in mastering is to arrange the songs on a project in the order they want to be in, and with the correct fades and spacing. This means listening to them in context, and repeatedly listening to the last 15 seconds and the first 15 seconds of each song to get a feel for how they transition. At the same time, listen to the relative levels of the songs. Meters don't tell the story here, especially peak meters. Perceived level is not represented well by meters, and is better measured by ear. Also, every song on a CD doesn't necessarily want to be as loud as possible, especially ballads. Another thing to listen for is large changes in frequency content between songs, the kind of changes that make a listener want to get up and adjust the EQ on their stereo on one song, and then want to adjust again for the next. Tonal and level consistancy is what we're trying to achieve, and if it's that the bass is mixed too high in one song, then don't cut the lows on the stereo file, go back and fix the mix. Only subtle changes should be necessary in mastering. If you have to grab a bunch of EQ to fix something, then you are almost certainly destroying something else.
There are some very good tools for mastering that are now available for home use, but they are not easy to use, and again, misuse will damage your mix. If you are willing to invest the time to learn them properly, plugins such as the Waves C4 compressor and L1 or
L2 Ultramaximizer can be very effective at punching up your master, and bringing the perceived volume up to compete with commercial CD's. The ultramaximizer is relatively easy to use, the Multiband compressor, any multiband compressor, is very hard to learn, and presets are almost useless. If you're willing to invest the time to learn them, they are powerful tools, but it will take time.
Ok, I've rambled on enough. Give the link that the Bear gave you a good read, it is excellent. Also, go to
http://www.izotope.com/products/audio/ozone/ozoneguide.html
and read the Ozone mastering guide, even if your not gonna use Ozone. It was written as a non software specific guide to mastering and is very good.
Best of luck, RD
I'm still butchering a lot of material. 
But I digress....