Lots of great advice in this thread.
One thing I'd like to comment on is that I have been lucky enough to have music I have mixed mastered by some top ME's. None of them reach for a multi-band compressor by default. They have all said to me, almost without fail, that the only time they pull out multi-band compressors is when something is really wrong with the track and there's no chance of the Producer/Engineer taking it back to fix it in the mix.
So it is a myth that Mastering Engineers typically use multi-band compressors but one I hear repeated a little too often.
Of course, they have their uses like everything else. Just thought I'd chip in for people just getting in to mastering.