One of the highlights of my Creative Writing class (for me, and hopefully, for the students, as well) is our daily writing exercise, at the beginning of class. I use the "What If" book to give them a creative prompt, and the students write on it for about 20 minutes, then share (if they wish).
Well, here are some wonderful sites I wanted to pass along, that contain lots and lots of creative writing prompts:
Creative Writing Prompts
Creative Writing Assignments that Work
10 Creative Writing Activities
(These are for elementary students, but I think some of them are quite thought-provoking): Journal Topics
So, when you're feeling stuck or unmotivated, you might skim these and see if they help "prompt" your creative juices to flow!