There is nothing harder than sitting down to write a song with no limits. Research has shown that the best creative thinking happens when there are strict limits put on the desired goal. Example: "Build me a widget that's really cool," will get less and weaker results than "Build me a widget that fits in the palm of my hand and enables me to know the weather for the next six hours."
So it is with songwriting. I wrote my first musical, and used this principle to make the songwriting really easy. I selected a four note musical theme, and every song had to somehow incorporate that theme. I could invert it, change from minor to major, break it up, change the meter, do a zillion things. The theme, though, had to create the melody. It made the whole musical part of the project really easy, and what's more, really interesting.
A song is like a beautiful flower. (Before you throw up, read on.) A flower starts as a blossom. The core is all there, but it needs to be unfurled. All the beauty is hidden in that blossom. So it is with a song.
Sammy Cahn once was asked which comes first: the melody or the lyrics? He answered, "The phone call." In his case, it was usually Sinatra calling for a new song about something in particular. That call, that something in particular, is the blossom. Sammy would come up with one phrase, and go to Jimmy van Heusen on the piano. Jimmy would put a small musical motif to the phrase. Then they could write the whole song.
That is the ultimate process of songwriting. Get a basic "seed" of a song., a melodic and lyrical phrase that fit together like glove in hand. It should encapsulate everything the song is about, and be a really engaging hook. (a hook is a phrase that gets stuck in your head - think "I did it my way".) From there, you need to do the harder work of unfurling that flower, of developing that song.
You've got your hook, your seed. That is your inspiration. Next comes your perspiration. Songwriting is hardest at the "filling it out" stage, in my opinion. That's why a powerful seed is crucial. You need to know what you are saying, and how to keep it fresh and surprising.
The art of fleshing out a song is the art of mapping. Before you get into the nitty gritty of each verse and chorus, bridge and coda, you need to know how they fit in to the overall journey you are taking your listeners on. Musically, the best map is one that increases the tension as you go, takes a slight breather just before the end, and then finishes the song in all glory, or in a fade to the starting level. It's up to you to keep it fresh and moving forward.
Lyrically, you need to plan what sentiments will be said in each part of the song. Avoid being repetitive, always add something new. New can be different, or the it can be the same, but with more details and insights. I'm not going to deal in depth with rhymes beyond urging you to shoot for the most natural way of expression. Never force a rhyme that sounds artificial. Listen to lots of Sondheim to see how a master does it!
A Marriage Made in Heaven
Finally, look at your song as a marriage between the music and the words. Are they congruous? Is the band wailing away while the lyrics are intimate and sensitive? Then redo something. Songwriters know how to match the music with the words in intensity, in mood and in meter. The words should roll off the tongue in song. Unless you are doing comic opera, the Gilbert and Sullivan tongu-twister is not recommended for contemporary music. Make it singable.
There you have it. In summary, get a great seed, develop it carefully and intentionally. Match the words with the music perfectly, and you will have your great song. See you on MTV, or wherever your dreams lead you!