a little Mid/Side processing trick i learned this weekend:
in addition to the method dos-ian mentioned above you can also do this:
use four channels of the same track to play with the M/S image.
double your track so its on two channels, each channel panned hard left and hard right.
then do it again and again so that you have four channels, two hard panned left, two hard panned right.
now take a LR pair and flip the phase. this becomes your SIDE pair, while the other is your MID pair.
now you can adjust the volumes of M and S independently, as well as EQ compress whatever each on its own.
if you've got the set up right, as you raise the faders on one pair, the mix will get wider and louder to a point, and then get quieter and quieter.
an interesting technique for widening during mastering, or bringing foreground stuff out a bit more,.... sometimes, but not all the time.