Using a single focal length does two things to refresh your creativity. First, it makes things simpler. You have much less to think about because you have fewer options. Secondly, it forces you to really understand what that specific lens is good for, and what parts of the scene it can best be utilized in.
Phototuts+ is a source of learning on all aspects of photography. We pump out regular tutorials on composition, equipment settings, post-processing, film photography, retouching and much more. If you love photography, you will find a treasure trove of useful advice at Phototuts+.