One of the most frustrating subjects for someone who is new to mixing music (or even intermediate), is how to deal with fighting frequencies. Fighting frequencies occur when you have 2 or more parts in a song fighting for the same frequency range. This causes one or both or those parts to lose their clarity, punch and impact. Some common examples are Kick drum and Bass, Guitars and vocals but conflict can happen pretty much with anything. Without properly addressing the problem you aren’t likely to end up with a professional sounding mixdown.
ps. If you're sidechain freak ;) then you can download 29 band sidechain compressor rack (for Ableton Live). Each band has it's own sidechain compressor available. This gives the ability to sidechain more than one frequency range from the instrument. Pretty clever :)
More info: Fighting frequencies and a 29 band sidechain compressor