IMO the only thing you should place on your master channel while mixing is some sort of spectrum analysis tool, like the one that comes with Live. This way, you can correct whatever bass/treble exess or dynamic issues you might be having in the final stages of mixing -
IF you can not already hear them. If your mix sounds bad/inadequate with no chain on you master track, read about mixing, not mastering

A mix should sound beautiful, and as close to a professional product as possible, before mastering. When A/B testing while mixing, make sure you're not cranking Timbaland or Skream way up in iTunes, while listening to yourself at typically low mixing volume - keep them fairly equal. Also, keep in mind that professional artists often mix and master in different ways for different places, i.e. a radio/myspace mix often has way more intense high mid than an album version, to reach the ears of passive listeners in a different way.
Do you use a pair of fairly good studio monitors? If not, bring your project somewhere where you can do all kinds of fine tuning before you send it off to be mastered. Other general tips; mix at a speaking volume, and occasionally bring it way down/up to check for balance issues you weren't percieving. If you're like most producers, you like to mix while composing (to get a 'feel' for the track), and your perception will suffer a bit from this. Don't make your final mix at the end of a long day, wait till the next day, you'll hear everything alot clearer

And, like others have mentioned, keep the mix peaking at -6db at least, and turn the volume up on your monitors/external mixer instead.
I am also in a learning process when it comes to this, so if anyone wants to elaborate on/argue with what I just wrote, please do!