Mastering is the act to finalize an audio project. It is the last step before the pressing/replication/duplication plant and the last chance to correct possible errors in the mix. Mastering is the opportunity to raise your work to its full potential. It will insure that it will properly translate, whatever type of equipment is used for playback.This objective work on frequencies and dynamics has to be done in an appropriate environment and mostly relies on the quality of the ears of the mastering enginneer and the perfect knowledge of his tools, the acoustics of the room, the monitoring system and the equipment used to treat the sound.If you feel your work needs to be improved or if you are preparing a project that needs to be sent to a pressing/replication plant, then it is a good idea to do it. Using a Mastering facility gives another advice on your work, another pair of trusted ears, another room, high-end audio tools. To keep an active communication with a mastering engineer is definitely the key to confidently finalize your project.You should already be really happy with your mix. Make sure it does sound in the way you want and keep a good tonal balance. Compare it with some other records you like, on different playback systems. Try to avoid(like the plague)limiter plug-ins or clipping on the master output during the mix. Leave some headroom: Let at least 3db between peaks and 0db. I recommend to use a k-20 meter during the mix, it is extremely useful to get enough headroom and to not worry anymore about level during the mix.Since too long and with the rise of the digital era, lot of people and productions think that mastering is all about making music LOUD and specially louder than the other(known as loudness war), but this is not necessary a good thing: Nowadays, Commerce laws and new technologies like the ipod(weak preamps),laptop speakers or mp3 dictate how the music standard should be, disregarding the quality. It shifts the role of the mastering engineer as a simple "make it loud" role, which is wrong. Mastering is the last opportunity to give a track or an album the final touch to reach its real potential and loudness is often just a little part of it, if not. For me it is far more important to make your work sound really good.I have generally experienced that over-squashed audio doesn't sound that nice(not natural, flat and no life in there!!!). On bigger playback systems, it often sounds thin compared to a good old dynamic tune. It generally adds lots of distortions and artifacts into a mix and it is painful for the ears.
People get generally fooled by the level: "it is loud so it is good", but if you compare side by side a squashed master and a dynamic one at the same level, there are no doubts about the winner. I personally prefer to have to turn the volume up because it sounds good than to have to turn it down because it is too painful.
Sometimes, artists or labels are expecting the mastering process to resolve everything and to raise a medium quality mix into a supermegahit! But mastering is not about doing miracles, it is 100% correlated with the quality of all what happened before this stage: (Recording, mics, console, preamp, gears, musicians, mix). The better this audio chain is, the better the master will come out. For example, a B grade mix will end up as B+ master or even A sometimes, B- mix will give a B master and so on. It is not possible to make the best sounding track ever with a C- mix. That is why your mix should be as perfect as possible before to be sent to a mastering facility.
Some links: