Repeat the same melodic idea over a changing chord progression. This can be great for building up tension.
The Long And Winding Road - verse
While My Guitar Gently Weeps - verse
Johnny Be Goode (Chuck Berry)
Many blues songs use this trick.
Ticket 9: Recycle Musical Cells
Ticket 13: Make The Vocals Stay On Non-Chord Tones
Ticket 14: Write An In-Key Melody Over Out-Of-Key Chords
Ticket 56: Write Your Melody Over Phantom Chords
Ticket 58: Write a Descant
See the full list of songwriting tips here - Tickets To Write