Quick reliable answer to this, is start on the foundation stuff, kick and bass guitar relationship, then check the bass and guitar relationship. This plus doing the basics like hi-passing/lowpassing instruments. Surgical cuts on the above relationships between the instruments. Another cool trick I do, is put your monitoring system as soft as you can and when playing back you can hear what's generally too loud or soft in the mix. All of this plus proper gain staging, then its all in the sample replacement, tight editing/playing and dynamics control. And make sure your master buss isn't clipping, if it is and there is nothing on your 2 buss then bring your channels and/or groups down. I also like to leave my master fader at -0.01db.
Hope this helps somebody