Bad mix good mastering can fix, but I have seen cases where bad mixing/recording and the mastering engr couldnt work with it. Im with Tom all the way "IF YOUR SERIOUS BOUT YOUR MUSIC GET A PRO INVOLVED" best ignored advice on the planet.
One question I've been wondering that doesn't really come up (or didn't at a quick glance); how "hot' should a mix be when it's sent in for mastering? Obviously you don't want it clipping, and I've been told it's a good idea to leave at least 6db or so of unused headroom for the ME to work with, but given the preference where would you mastering guys want a mix to peak? -6db? -12? Does it even really matter, as long as it's below 0?