Compression and EQ ( although not necessarily in that order )
Lots of thing to do for this one. If things are fighting for space, look at you panning, look at you MIDI data ( do all those notes need to be unison?? ) look at your efx, ( does everything need a stereo reverb? )
Can I cut some 'treble' out of one thing without losing anything?
There's lots to talk about and really, people will be just giving a huge list of things ( mine was just the tip of the ice-berg )
Why not post a clip of your track, no need to post the whole track, just the bit where you are particularly concerned about.
Regards.