Coming at the question from the direction of writing (mostly) comic songs:-

Inspiration, inspiration, inspiration!

You have to observe something which is genuinely funny, and the rest is word juggling.

Serious songs now, well that's much more difficult. I always respond best when the composer has made an effort to avoid all the old cliche phrases. So that's what I try to do.

Do I succeed? You'ld have to ask someone else, as I don't feel qualified to judge.

I DO get requests tho', so maybe......

Don T.