I most often put up a rythm on my Alesis SR-16 Drum machine and then sit down with a piano sound on the keyboard. The tempo is often at this stage around 73 bpm. Then I just start to play some chords. After I have a progression (much too often 4 chords) I start to sing to it, singing the first words that drops in. Just nonsense. After some repititions I start varying the melody line to try to find a better one. I allways start with the verse. Then, at that point or maybe later, I will get a feeling for if it should hold its tempo or be more uptempo. If the latter I crank it up to 90-100 bpm, and sing it for some rounds again.
If Im satisfied with whats happening I try to make a pre-chorus. The prechorus normally just drops in within a minute or two.
Then I try to find an interesting approach to a chorus.
I often end up with using the verse in one song as a chorus in another one, after changing it to suit the chorus formula a tad more.
As I see it, its better not to listen to something else in the last 30 minutes before you start writing.
The most tricky thing about songwriting for me now is to try to avoid the mainstream chord progressions and think different/new. But then again, a lot of hits has standard chord progressions, some have not.
Its a melting pot of brilliant elements that forms your next hit
On a side note; a friend of mine allways starts with the chorus, and he can NEVER sit down and play unless some catchy chorus came to him during his time doing anything else than playing his own music. When he gets an everyday chorus idea on a visit he starts writing.
We are meeting friday and I said, hey, lets make something together friday. He replied:
I cant. I dont have any chorus ideas. hehe
I will get him going.
Good luck!