i dont master my own stuff but if I did...
a quality master bus compressor is what really makes the difference for me, the X rack G series compressor (i think thats what its called) works great for "glueing" the sound together.
mastering is more than compression tho, different guys do different stuff. there is multiband compression (theres the C4 plugin that does this, which does not sound good to me, and theres also hardware that does this:
http://www.tube-tech.com/?show=produ...er&productId=1)
Eq feathering, stereo difference eq-ing de-essing, low-end mono summing (for a plugin that does this check out
http://www.brainworx-music.de/index....2&um=2&lang=en)
mastering guys generally listen on different monitor speakers than mixers and recording engineers, and also many of them also use extremely high quality A to D converters like the Lavry MK111 (which isn't cheap, they run from 8-10 grand)
I'm definitely no expert on it, but you can get an idea of what kind of equipment mastering guys use by just asking, a lot of guys will tell you exactly what they use and do. Its the whole philosophy of "its the driver, not the car" :)