Bass is the big monster in your mix that fills up a whole lot of your dBFS meter on the master buss. Bass can be a troublesome foe in your mixing quest, but do not fret! Bass has one big weakness, High-Pass Filters.  

High-Pass filters are mysterious to some mixing engineers, but not to us. High Pass filters can do wonders for a dense mix that has lots of different tracks. When your mix is too boomy or muddy, first try using some high pass filters on the tracks that don't necessarily need to have low bass frequencies. Rolling off the low end on vocal tracks will really help separate the vocals from the rest of the mix. This may also immediately shape up the low end of the entire mix. Go through each track and use your ears to attenuate the HP filter on each track for a more controlled bottom end to your mix. You may not even have to touch the bass track or the kick track.

Alternately, you should use Low Pass Filters on tracks that reside in the low end of the mix. Rolling off the high frequencies on the bass and kick tracks may open up more brilliance and space in your mix.  It's not too complicated, but it does take a little bit of practice to get used to using filters to control the low end of a mix. Be sure that you adjust the filters with the tracks playing in the mix. If you solo the track while you adjust the filters, you may not get the tone or control that you need when the tracks are put back into the mix. Soloing has it's place, but when you're trying to get tracks to fit together in a mix, it's best to adjust them while they're in the mix.