Being creative doesn't mean spending hours agonising over a single idea. In fact quite the opposite. Forcing yourself to work under a timer can often achieve better results, much like working to a really tight deadline. Some of my finest design work has been created when it felt like it was physically impossible to achieve. The kind of hear-your-heartbeat-in-your-ears kind of stress.
Putting yourself under false deadlines will help push out different kinds of imagination and also improve your ability to get across a really good idea in a short amount of time. For instance for a pitch or if you really want to kick-ass at Pictionary.
Here's a list of ways to boost your flow of creativity and help improve your portfolio, just make sure you have a watch/timer handy and go for it.