Hmm, nobody replied to this yet? Guess I will then. I'm certainly not an expert at song writing, but I've written a few of them.
The question is, are you writing the music part, or just the lyrics? If you are writing just the lyrics, is the music being written before or after? In my experience it's a totally different process whether you're creating words to fit music or just creating words which someone else will fit the music to.
If you do have music, I would suggest listening to it several times with your eyes closed. If you don't have music you want something else with a beat to listen to -a metronome, a keyboard or drumpad which can produce simple automatic rhythms, sound effects of a train or a galloping horse or windshield wipers... writing while listening to a rhythm prevents the problem of writing a poem which refuses to be set to music.
Other than that, follow standard poem writing procedures, keeping in mind that sung rhymes are more forgiving than read rhymes, and you probably want to have a chorus with words that resonate in different witty ways in the changing context of the verses around them. To avoid musical boredom, make sure the meter of the chorus is different from the meter of the verses.
