Production Tip: Mixing 101

Switch between monitors and headphones to balance out your mix so you like how it sounds equally on both.

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

If something is too quiet, dont turn it up -turn everything else down.

listen at low to mid volumes when mixing. do not crank the volume all the way up to "feel" the music

Take breaks, to rest your ears.