I'm so used to working against requirements and deadlines set by others that I constantly have problems setting my own deadlines for my own projects. Once I start an image or a project it's easy to keep moving forward. I'm excited by seeing the progress being made and that keeps me motivated to continue. But as I finish, and don't have another idea ready to go from brain to page, I find the wheel of progress grinds to a halt. I've been trying to kick start it up again by doing my usual - look at other artists and what they're doing, flip through the art books of my favorite illustrators, skim-read my novels. Maybe I'll find an older sketch and bring that up to date.
I'm sure I'll think of something soon...sometimes just talking about being uninspired helps overcome that block.