cut frequencies below 150hz-200hz on ALL instruments that are not bass.
cut frequencies below 30 hz on bass.
cut resonant frequencies on vocal tracks.
Always keep a/b'ing your changes to make sure they are for the better.
To add to what tactik said..... Have as many possible kinds of reference as possible..
ie headphones, hi fi speakers, small laptop speakers/cheap full range speakers, studio monitors, in the car etc etc