When designing, it's very easy to get stuck on a specific direction or style. I often find myself "working" on a project, but end up just going in circles and not making any real progress. In most cases I find that the more I work on a design, the more I tend to overwork and over think it. It's easy to get wrapped up the fine details without looking at the design as a whole and considering all of the options.
The best advice I ever heard, though it may seem obvious, is to "sleep on it." It's amazing what can happen when you step away from the sketchbook and computer and give it a good night's rest. Unfortunately it doesn't magically work itself out while you're gone, but it does allow you to approach the design with a fresh perspective which often triggers new ideas and concepts. So if you're ever stuck on a design, illustration, or any type of creative project, drop the pencil and mouse, take a break, and sleep on it.