I like to work from photos, too, for landscapes. Often, putting two or more together will make a better composition. I have pictures of many random barns, buildings, trees and other things waiting for just the right moment.
After years of teaching and producing murals, borders and vignettes on walls for my clients, painting on demand is often the way it has to happen. When you have to do it now, the inspiration has to come from somewhere, so I usually just take a few minutes and concentrate on the task at hand and begin running ideas through my head until the juices begin to boil.
If it is for a client, talking it over with them & bouncing some thoughts and ideas around can really help. At that point, a quick sketch, usually with chalk, is what firms up the composition for me.
I think the most important point is in the planning and sometimes just BEGINNING can be what makes the difference. If I don't start, I'll never finish!