In a busy mix, try “ducking” midrange instruments such as overdrive guitars and synth pads under the control of the vocals, so that whenever the vocals are present, the conflicting sounds fall in level by two or three dBs. Just a little ducking can significantly improve the clarity of a mix. Use a fairly fast attack time for the ducker (which may be either a compressor or a noise gate that has ducking facilities), and set the release time by ear. Shorter release times will cause more obvious gain–pumping, but in rock mixes, this can add welcome energy and excitement.
From time to time, check your mix balance by listening from outside the studio/bedroom door. This tends to show up level imbalances more clearly than when listening from directly in front of the monitors. Nobody is quite sure why, but it works.