Rhyme, as many have already said is a bit "take it or leave it" but RHYTHM is KING!! and if Possible(a rare luxury if you are using original tunes) have the Tune in your head or even put the number of feet on the paper your are writing the lyrics on ( dah dum dum Dah dum dum Dah dum) and then fit the words to the rhythm. Some times rhymed words establish their own rhythm ,but non-rhymed really need you to be aware of the underlying rhythm which can entirely change the snse of the poem.
an example
Awake for morning in the Bowl of night
Has flung the stone that puts the stars to flight
and Lo! the hunter of the East has Caught
the Sultans turrett in a Noose of Light
Omar Khyyam trans Fitzgerald
Now try the same words to the tune of "Hernandos Hideaway"
Finally ALWAYS carry a notebook!!! when the Muse strikes , BE READY,