For me it is “automation”
I like to get my mixes to what I refer to as a “static mix” - ie all processing (EQ, compression, fx etc.), volume balancing, and panning so that the whole track sounds great
Then I will listen for those special little things (or I will have noted them down previously) that I want to bring out in the track - eg a little piano flourish, a bass run, a drum fill, a guitar riff etc. and use automation (usually just a little volume lift) to gives each element a little moment in the spotlight
I keep it VERY subtle but it definitely adds a bit of interest to the track and means the listener usually picks out something different on each listen
I also like to use automation on a bigger scale to give a subtle difference to each verse or chorus - eg in verse 1 I might keep the piano loud and guitar quiet and then in verse 2 I will bring the piano down and boost the guitar - keeping things subtle
This keeps the “feel” of the song consistent but there is a subtle variation to maintain interest