A simple approach to mixing a rock song: Part 4 - Equalization (What is it?)
Panning is one way to separate clashing instruments. Equalizing is another. Equalization changes the amplitude of various frequencies in the sound mix. Some people even swear that equalization creates the perception of moving instruments up and down in the mix (as opposed to panning, which moves them left and right). Is that even possible? Perhaps. Having different instruments occupy different frequency ranges definitely makes them more distinct in the mix.