Compressing your mix makes the individual tracks take up less dynamic space without losing their sonic impact. As you begin your mix, start conservatively with your compressor settings: the attack and release should be relatively fast, set the ratio at 2:1 or 3:1, and set the threshold so that only the peaks are getting over the top and being "squeezed." You’ve probably heard the phrase, "Punchy Mix." Stereo compression is a key element in making your mix more "punchy." EQ’ing the stereo buss – the two wires that funnel all the various sounds from your mixing console to your 2-track recorder – is generally discouraged. Since this eq affects the entire signal, from your bass guitar to your lead vocal and everything else in the mix, you’re likely to affect some sounds you didn’t intend to. For example, if you decide that the mix is lacking bottom end, try adding a little here and there on the bass, the kick, toms or whatever needs it.

If you add…

Source: http://ezinearticles.com/?The-Art-of-Audio-Compression-(Pro-Audio-Tips)&id=1243730