I don't process the master much. There are certainly loudness maximizer plugins and the like, but you are shortchanging yourself if you resort to those imho. I find that adding reverb to the master doesn't accomplish much for me, but that's probably because I add individual reverbs to each of the tracks I want reverb on. About all I put on the master is compressor that works at about -6 at the loudest parts, and it gets smashed into a limiter. I find it best to smash the whole mix into compressor->limiter from the start if achieving maximum loudness is what you want. I'm sure there are people who disagree with me but whatever.