just load your mixed down song in your program as a stereo wav. then add > Eq, Compressor, and Dither...
play with different levels... I recomment like a 5:1 comp ( and some attack) and just eq's it tell it hits, and the dither will give it a higher quality bit rate (artifical good sound)
that is poor man mastering, and does more then most artist do to there cd's now days