Listen to a great creative player such as Shaye Cohn and note how frequently she gives a lot of emphasis to 6ths and 9ths - especially at the start of one of the final choruses, when she is still finding fresh approaches. For example, a tune may begin with two bars firmly on the C major chord but you may find Shaye decisively hitting several 'A's (the 6th). Or in a tune beginning with the D major chord, she will deliberately and firmly go for the 'E' above the chord, making a 9th.
Another is If I Had You: Think of it in the key of Bb. Numerous times in this song, you find (yes - in the composed melody) a C being played above a Bb chord. You find a G being played above an F7th chord. And you find Cs being played above Eb chords.
I'm Crazy 'Bout My Baby is another. Its Middle Eight is unusual, its special effect being achieved by the fact that the melody notes are so often the 6ths or the 9th of the chords.
Effective, isn't it?
Reader Barrie adds: