When picking chord progressions to write over, pick ones from songs you love and that move you. Much of the power of the chord progression will often transfer to your melody. The chords will have a certain color to them regardless of what melody notes get sung over them.
Writing over new chords can also help you get out of any melodic ruts you may be in. A chord progression will naturally limit the number of notes, because only three or four notes will work over each chord.
If you write over the same chords all the time, you’ll have a tendency to fall into melodic patterns. This explains why so many blues melodies sound similar to each other.
Writing over a new chord progression can help you breakout of stale melodic patterns. Writing over chords from a style you don’t usually write in can push the process even farther.
Similarly, if you write in verse/chorus form all of the time, try writing in AABA form every third or fourth song. Getting into ruts can lead to stale writing. Therefore, breaking out of ruts can help jumpstart the creativity in your songwriting.