"What I have most wanted to do throughout the past ten years is to make political writing into an art. My starting point is always a feeling of partisanship, a sense of injustice. When I sit down to write a book, I do not say to myself, ‘I am going to produce a work of art’. I write it because there is some lie that I want to expose, some fact to which I want to draw attention, and my initial concern is to get a hearing. But I could not do the work of writing a book, or even a long magazine article, if it were not also an aesthetic experience." - George Orwell, "Why I Write"
The emotional inspiration (for Orwell it is the injustice) must be present. You cannot sit down and say I want to write a song and compose brilliance out of thin air (although there is a good deal of luck with brilliance). Eating well and staying physically healthy will surprisingly improve your inspiration results: for me to write a song (or a poem) I always need emotional inspiration, and I cannot handle such excess levels of feeling--or handle them for the entire time of writing, it is surprisingly exhausting--without having enough energy. Robert Hunter also had what he called an "unconscious memory": an ability to resume the emotional state of inspiration from a night before. We don't always have an instrument/paper at hand for when those moments come (and if you are getting your inspiration from life then it surely won't be there at the time--unless you want your life itself to be the song) and need to preserve the feeling somehow until we can get it into our instrument, to which it is then preserved in song.
This is very general advice, but I'm sure others will talk about how they start with a motif and then begin expanding on it.