Still working on this one - I tend to paint and then leave a work sitting around the studio, letting it stew. Something might keep bothering me while I keep catching it from the corner of my eye. The trick is to wait patiently until I have an idea for it, usually when I'm busy doing something else. I have in the past just been stimulated into just going at it. Not sure of the success of this method yet. I've painted over things and then seen a photo of it later and thought 'oh no, that was actually quite good'.