Sing everything.

[Aaron Aldous]

Feeling like you want to write something? Sing it.
Transcribe a part? Sing it.
Learn a scale/arpeggio/interval? Sing it.
Play over changes? Sing over changes.
Improve your connection between your inner ear and fingers? Sing everything you play, and play everything you see.

I’m yet to find a musical situation where this hasn’t applied at some point.

 

[SimonEichinger]

This! Amazing for improv. Especially when playing over changes, as you said.