Sometimes when you're caught in a bit of a drawing or writing rut you can't help but go over and over things in your head. I was reading a post on one of my favourite non drawing blogs, a fantastic blog on the process of innovation, and I came across this quote,
"...build a bias towards action, not thinking"
When you feel stuck it helps to get the pen moving, just get started on something anything. If you have new ideas, try the new ideas. Be ready for failure too, that's ok, but don't be scared to just jump straight in and try out some new things. This is one of the many reasons I think sketchbook practice is important, it helps you get out of your head and act on ideas, and you never even have to show anyone if you don't want.
In short, if you ever get stuck, just keep on moving.