in addition to using one instrument to trigger the compressor of another, you can run a mix through the compressor, and stick an eq in the sidechain, allowing you to compress the whole mix based on the frequency content. for example, buy cutting the bass on the sidechain eq, you make the compressor respond more strongly to high end transients, but respond less strongly to low end transients. good for keeping the high end from jumping out every time the bass drops out, which can happen when you use too much compression.