Check out this free RTAS plug-in for spectral analysis:
http://www.elementalaudio.com/products/inspector/
I just started using this for a project that I'm self-mastering and can attest that it's very helpful to see the frequency ranges. For me, it's a lot of trial and error and slow progression to train my ears. I'll usually do an initial master, burn a CD and listen on a variety of stereos. Usually I find that my mixes are too muddy and too heavy in the low frequencies. The Spectral Analyzer really helps me SEE this before my final bounce. I'm FAR from really understanding the details of mastering, but I found this tool helpful.