For me it's writing against different rhythms. I write a lot of songs across a wide range of genres. Even genres I don't particularly like but it's a good exercise. Even if the chords are the same different rhythms generate different melodies. Also experiment with different song formats like instead of writing the song with the verse first start the song out with the hook first. That can make a change in the way you write the song because it requires a different delivery of the melody. Those are just a couple of suggestions.