Taking time off or away from a project is really important. That becomes really clear to me the more I refuse to do so. If I work on something for too long with no breaks I can't be objective. I get too close to it and have a hard time coming up with ideas.
I have to take time for my brain to work on it. And you know...a lot of the time the solution presents itself subconsciously in a dream. I go to sleep, sometimes not even thinking about the problem, and I dream about the answer. The key visual to a new illustration project... a solution to a problem I had with a relationship... etc. My brain just needed to work on it without my knowing it.
I also find that I feel most creative when I allow myself to step away. Take a walk, go to the bookstore, watch a movie, talk to a friend. It's like my creative well needs to be refilled. And the whole world is out there just waiting to pour in.