A couple of things I use here when i'm mastering your finished tracks are processes that glue the elements of the mix together. For many years i've actually run the mixes out via our Apogee Rosetta DAC into the analogue desk, placed our R3 Focusrite compressor in stereo mode across the mix insert, compressed maybe 1-2 db off the peaks then back into the digital domain for further processing. It helps the bass and kick drums stick together and gives a heavier but tigher bottom end to the mix without destroying the dynamics. I've also got a hardware G series SSL buss compressor that I use in the same way.
Now i'm increasingly using the McDSP Analogue Channel plug in to do the same thing and I have to say i'm really pleased with the tine i'm getting from it. I'm pretty much using the tape simulation part of the plug at the moment, and most often on modern and Swiss or US A settings. I'm interested to hear other mastering engineer's opinions on using this plug in.