Decide what should stay alive
Use compression when a part jumps too far in level, or when its quieter details need to stay present without losing every transient.
Pick a model by its job
Clean and VCA hold a part steady, FET grabs fast transients, Opto smooths vocals and bass, Vari-Mu glues a bus.
Hear it against the song
Set the response in Rack while the complete groove plays; key it from another track when two parts compete.